GEOLOGY 

CHAPTERS   OF   :  ./ :  7H   HISTO 


.EORGE  HICKLING    M.Sc. 


UC-NRLF 


SB    277 


XXth  CENTURY  SCIENCE  SER1 


GEOLOGY 

Chapters  of  Earth  History 


WHIRLPOOL"  NEBULA. 


[Keeler. 
M.51  Canum  Venaticorum. 


Reproduced  by  permission  of  Professor  Campbell  from 
Publications  of  the  Lick  Observatory. 


Plate  I.} 


[Geologv,  frontispiece. 


GEOLOGY 

Chapters  of  Earth  Hiftory 


GEORGE   HICKLING,   M.Sc., 

N\ 

Ledturer  in  Palaeontology  and  Assistant  Ledlurer  in  Geology 
in  the  University  of   Manchester. 


NEW  YORK 

FREDERICK  A.  STOKES  COMPANY, 
PUBLISHERS. 


CONTENTS 


PAGE 

INTRODUCTION        ...            ...            ...            ...  1 

CHAPTER  I. — THE  ORIGIN  OF  THE  EARTH     ...  5 

II. — VOLCANOES  AND  EARTHQUAKES...  25 

III.— THE  SOLID  ROCKS      ...            ...  42 

IV. — EARTH  SCULPTURE      ...            ...  55 

V.— THE  SEA-FLOOR          ...            ...  70 

VI. — MOUNTAIN  BUILDING  ...            ...  81 

VII. — THE  PHYSICAL  HISTORY  OF 

BRITAIN        ...             ...            ...  97 

VIII. — THE  HISTORY  OF  LIFE  ON  THE 

EARTH  116 


280864 


The  Publishers  are  indebted  to  the  following  for  per- 
mission to  reproduce  illustrations  in  this  volume  : — 

Professor  Campbell  -  -  "  The  Whirlpool  Nebula." 
Dr  Tempest  Anderson  -  "  Eruption  of  Alt.  Pelee." 

Mr.  R.  Geiler  -  -  "  Atemir,  near  Settle." 

Dr.  Bartholomew  -  -  "  Section  of  the  Grampians." 
Trustees  of  British  Museum — "  Skeleton  of  Brontosaurus," 

from  the  sketch  by  the  late  Prof.  O.  C.  Marsh  ; 

"  Development  of  the  Skull  of  the  Elephant." 
Messrs.  Macmillan  &  Co.,  Ltd.—11  Development  of  the 

Limbs  and  Teeth  of  the  Horse,"  from  Huxley's 

American  Addresses. 


LIST  OF    ILLUSTRATIONS 


Plate  I.—  The  "  Whirlpool  "  Nebula.    M.51  Canum 

Venaticorum  ...  ...  ...  Frontis. 

I  1.  —Eruption  of  Mt.  Pelee,  Martinique,  1902 

facing  p.    28 
III.—  Atemir,  near  Settle       ...  ...  ...     „         59 

IV.  —  Typical  Fossils.  1,  Trilobite  crustacean  ; 
2,  Brachiopod  ;  3,  Cephalopod  ;  4,  Echi- 
noid  ;  5,  Gastropod  ;  6,  Pelecypod  ...  ,,  116 

Fig.  1—  Ideal  Section  of  a  Volcano    ...  ...  ...    £31 

2—  Aughros  Head,  Co.  Sligo       ...  ...  ...       41 

3—(A)  Thin    Slice   of   Granite   from    Dalbeattie, 
Scotland.    (B)  Thin  Slice  of  Rhyolite  from 
Elfdalen,  Sweden  ...  ...  ...        45 

4—  Lion  Rock,  Isle  of  Cumbrae,  Clyde    ...  ...        51 

5  —  (A)  Portion   of    course   of    meandering    river. 

(B)  Course  of  same  river  at  later  period, 
changed  by  erosion  of  banks  and  deposit  of 
sediment  ...  ...  ...  ...  68 

6  —  Fold  in  rocks  forming  cliff  west  of  Little  Hang- 

man Hill,  near  Ilfracombe  ...  ...        89 

7  —  Section  illustrating  the  gentle  folding  of  the 

rocks  in  the  South-east  of  England  ...        91 

8  —  Section  illustrating  the  intense  folding  of  the 

rocks  in  the  Grampians  ...           ...           ...  91 

9  —  Rocks  on  shore  at  Stccar  Point,  Berwickshire  101 
10—  Geological  Map  of  the  British  Isles  ...            ...  104 

II  —  Skeleton  of  Brontosaurus,  a  Deinosaur,  from 

the  Upper  Jurassic  of  Wyoming  ..,  ...      125 

12  —  (A)  Development  of  the  skull  of  the  Elephant. 
(B)  Development  of  the  limbs  and  teeth  of  the 

Horse    ...  ...  ...  ...  ...      130 

vii 


INTRODUCTION 


THE  tale  has  gone  the  rounds  of  the  aged  Abbe" 
who,  being  accosted  wandering  alone  among  the 
mountain  wilds,  answered  the  inquiry  how  he  came 
to  be  there  with  the  story  of  a  dream  he  had  had 
during  the  course  of  a  fever  which  he  had  believed 
to  be  mortal.  In  his  vision  he  was  asked  by  his 
Maker  what  he  thought  of  the  beautiful  world  in 
which  he  had  been  permitted  to  live.  The  question 
was  a  revelation  to  him.  He,  who  had  spent  his  life 
exhorting  his  fellows  to  look  to  the  beauties  of  a 
future  world,  had  never  thought  to  look  around  him. 
He  could  make  no  answer.  But,  when  he  awoke,  he 
made  a  vow  that  if  he  should  still  be  permitted  to 
live  he  would  devote  his  grey  years  to  an  inspection 
of  the  world.  Health  returned,  and'so  he  was  found 
upon  his  pilgrimage. 

The  story  has  become  a  favourite,  because  few  can 
read  it  without  sympathy.  How  many  could  give 
account  of  the  beauties  of  the  world  in  which  they 
live  ?  It  is  recorded  here  because  it  expresses 
eloquently,  if  unconventionally,  the  import  of  that 
much-misunderstood  word,  Science.  For  it  is  the 
whole  object  of  science  to  know  the  beauties  of  the 
world  around  us  ;  not  only  the  world  as  it  is  to-day 

1 


INTRODUCTION 

but  as  it  has  been  in  the  past,  also.     It  is  the  special 
province   of  Geology  to   recover  the  story  of  the 
earth's   past,  to  show  through  what  vicissitudes  it 
has  attained  its  present  condition.     In  all  ages  and 
all  climes,  from  the  earliest  dawn  of  human  civilisa- 
tion, man  has  sought  to  know  how  the  world  came 
to  be.    Not  only  had  the  Jews  their  story  of  Creation, 
but  similar  legends   are  found  among  the  oldest 
writings  and  traditions  of  many  nations  of  antiquity. 
Whether  Geology  or  Astronomy  is  the  older  science 
can  never  be  known,  both  having  their  roots  deep  in 
the  lost  grounds  of  early  human  history.     Indeed,  in 
the  earlier  ages,  they  were  not  two  sciences,  but  one. 
Cosmogony  was  alike  the  foundation  of  them  both. 
Various    causes  during  the  chequered  history    of 
science  led  to  their  gradual  separation,  notably  the 
recognition  of  the  true  place   of  the  earth  in   the 
universe,  and  in  modern  times,  the  intense  reaction 
against  the  wild  speculations  of  the  Middle  Ages, 
which  led  geology  to  repudiate  all  concern  with  the 
origin  of  the  earth,  and  to  confine  her  attentions  to 
its  history  since  it  became  a  globe  such  as  we  now 
know  it.      But  the  step  was  a  mistaken  one,  and 
geology  is  rapidly  returning  to  her  old  alliance.    She 
has  been  true  long  enough  to  her  old  watchword  that 
the  Present  is  the  key  to  the  Past,  and  is  beginning 
to  realise  again  that  it  is  even  more  profoundly  true 
/that  the  Past  is  the  key  to  the  Present.    Yet  the 
exigencies  of  our  position  compel  us  always  to  work 
from    the    present    to    the    past;    and    while    the 


INTRODUCTION  3 

astronomer,  by  observing  the  present  condition  of 
other  worlds  throws  light  on  the  past  of  our  own,  it 
is  by  carefully  noting  how  changes  proceed  around 
us  to-day  that  we  are  able  to  interpret  the  changes 
which  have  clearly  occurred  in  the  past.  Thus, 
occupying  an  intermediate  position  between 
Astronomy,  with  the  unfathomable  universe  for  its 
fields,  on  the  one  hand,  and  the  many  sciences  which 
seek  to  investigate  the  varied  manifestations  of  life 
and  matter  in  our  own  world  on  the  other,  Geology 
seeks  to  apply  the  truths  gathered  by  them  all  to  the 
interpretation  of  the  past  history  of  the  earth  record- 
ed by  Nature  herself  in  the  rocks  at  our  feet.  It  is, 
therefore,  the  most  comprehensive  of  all  the  sciences. 
To  the  geologist,  the  world  of  to-day,  with  all  its 
varied  aspects  of  being  and  doing,  is  but  a  moment- 
ary glimpse  of  Nature  in  her  grand  progress  of 
evolution  through  illimitable  time. 

Tlae  foregoing  indication  of  the  wide  scope  of  our 
subject  should  sufficiently  indicate  why  we  have 
chosen  to  entitle  this  little  volume  "  Chapters  of 
Earth  History."  So  manifold  and  so  diverse  are  the 
branches  into  which  the  study  of  geology  divides 
itself  that  no  volume,  however  large,  can  give  a  com- 
plete account  of  the  whole,  even  in  outline.  Nor  is 
a  systematic  account  our  intention;  but  rather  to 
glance  at  the  subject  from  various  points  of  view, 
so  that,  without  detaining  ourselves  for  a  minute 
inspection  of  any  aspect,  we  may  gain  a  general 
impression  of  the  whole.  As  we  have  seen  that 


4  INTRODUCTION 

speculations  concerning  the  origin   of    the    world 
attracted  attention  ages  before  any  other  geological 
theme,  we  may  believe  that  the  same  question  will 
still  afford  the  surest  foothold  for  the  interest  of  the 
general  reader.     Following  a  brief  account  of  that 
subject,  we  must  see  how  the  rocks  around  us  may 
be    made    to   disclose    their    history,   watch   them 
crumbling  under  the  beat  of  wind  and  rain,  see  their 
remains  buried  under  the  sea  and  raised  again  to 
form   new  lands.     We   shall   see   in  the  incessant, 
though  imperceptible,  heavings  of  the  earth's  crust, 
the   power  which   reproduces  the  lands  which  the 
elements  destroy ;  in  the  volcano  and  earthquake  its 
visible  manifestation.   In  the  concluding  chapters  we 
will  endeavour  to  trace  something  of  the  history  of 
our  own  country  and  of  that  strange  succession  of 
extinct  creatures  which  peopled  the  world  in  past  ages. 
The  chief  hope  of  the  writer   is  that  this  brief 
sketch   may  do  something  to  add  to  the  reader's 
interests  by  enabling  him  to  find  greater  meaning  in 
the  scenes  around  him.     The  great  merit  of  geology 
is   its  power  to  attract  its  followers  out  into   the 
country,  to  find  food  for  the  mind  as  well  as  exercise 
for    the    body    among    the   sea-cliffs   and    on    the 
mountain-side.      However  delightful   the   works  of 
the  great  pioneers  of  the  science  may  be  to  occupy 
the  leisure  hours  at  home — and  they  form  no  mean 
contribution  to  the  literature  of  the  past  century — 
it  is  in  the  field  that  they  acquire  their  full  meaning 
and   their    fit    surroundings.      Probably   no   other 
science  is  so  well  calculated  to  form  a  healthy  and 
manly  hobby;    none,  certainly,  is  better  fitted  to 
enlarge  one's  perception  of  the  beauties  of  Nature, 
to  lead  one  to  a  truer  realisation  of  the  littleness  of 
human  concerns. 


GEOLOGY 

Chapters   of   Earth    History 
CHAPTER  I 

THE   ORIGIN   OF  THE   EARTH 

OUR  knowledge  of  the  true  place  of  the  earth  in 
the  universe  has  been  acquired  only  after  many 
centuries  of  patient  search.  Even  so  far  back  as 
the  time  of  Hipparchus — perhaps  long  before — men 
had  realised  that  the  earth  was  a  spherical  body. 
The  constantly  circular  horizon  at  sea,  the  circular 
shadow  of  the  earth  whenever  it  was  seen  projected 
on  the  moon  during  an  eclipse,  did  not  hide  their 
meaning  from  the  ancients.  Some  fair  approach  to 
accuracy  was  attained  in  the  measurement  of  the 
diameter  of  the  earth  nearly  two  thousand  years 
before  it  was  destined  to  be  circumnavigated. 
Measurement  of  the  apparent  height  of  the  sun 
above  the  horizon  in  different  parts  of  Egypt  at  the 
same  time  provided  the  necessary  data.  For  the 
apparent  elevation  depends,  of  course,  on  the 
latitude;  hence  if  it  is  found,  for  example,  that  two 
places  (one  directly  north  of  the  other)  are  1  degree 
apart,  and  the  distance  in  miles  be  measured,  mere 
multiplication  of  the  latter  distance  by  360  is  required 
to  give  the  circumference  of  the  earth,  assuming  it 
to  be  really  a  sphere.  Even  the  distance  of  the 
moon  was  approximately  known  to  the  Greeks,  and 

5 


6  GEOLOGY 

consequently  something  of  the  size  of  that  body. 
Hipparchus  himself  devised  a  method  for  estimating 
the  distance  of  the  sun.  The  difficulty  of  the 
necessary  observations  (on  the  length  of  the  various 
phases  of  the  moon)  defeated  his  aims,  though  he 
was  enabled  to  conclude  that  the  distance  could  not 
be  under  3,000,000  miles,  and  the  sun,  therefore,  a 
body  many  times  larger  than  the  earth.  Ages  earlier, 
the  ancient  star-watchers  of  the  East  had  picked  out 
those  five  "  stars  "  which  are  distinguished  from  all 
the  hosts  of  the  sky  by  their  wandering  habits, 
changing  their  places  night  after  night  among  the 
true  or  "  fixed  "  stars.  Observation  even  permitted 
them  to  conclude  that  two  of  these  "planets"  (Greek 
— a  wanderer),  viz,  Venus  and  Mercury,  were  bodies 
which  circled  round  the  sun,  and  were  therefore  at 
times  nearer  than  that  body,  while  the  remaining 
three,  Mars,  Jupiter  and  Saturn,  were  more  distant; 
and  that  furthest  of  all  were  the  stars.  But,  having 
gone  so  far,  human  prejudice  barred  the  way  to 
further  progress;  no  mind  was  yet  free  enough  to 
escape  the  conviction  that  the  earth  must  be  the 
centre  of  the  universe.  Whatever  advance  had  been 
made  towards  a  true  estimate  of  the  relative  dimen- 
sions of  the  earth  and  the  space  and  bodies  around 
it,  the  essential  belief  still  remained  that  this  globe 
was  in  the  fullest  sense  the  centre  of  all  creation. 
Hence  arose  those  complicated  schemes  which 
sought  to  explain  the  observed  motions  of  the  sun, 
moon  and  planets  on  the  assumption  that  they  all 
moved  round  the  earth.  It  can  scarcely  be  necessary 
to  record  how  this  "  Ptolemaic  "  system  of  astronomy 
was  implicitly  believed  for  upwards  of  fifteen 
hundred  years,  until  Copernicus,  in  1543,  announced 
his  discovery  that  all  the  observed  phenomena  were 


THE  ORIGIN  OF  THE  EARTH  7 

explained  quite  simply  if  it  were  assumed  that  the 
earth  itself  is  a  planet  revolving  annually  round  the 
sun.  The  story  of  the  persecution  of  the  followers 
of  Copernicus  for  this  appalling  "heresy"  is  only 
too  well  known,  but  it  is  worth  recalling  in  order  to 
emphasise  the  real  magnitude  of  this  step  towards 
the  realisation  of  the  earth's  true  place.  The  earth 
was  "  degraded  "to  be  a  mere  satellite  of  the  sun, 
and  but  one  of  six,  at  that. 

Now  this  great  barrier  was  passed,  progress  was 
rapid.  It  was  soon  possible  to  draw  a  map  of  the 
"  Solar  System,"  showing  the  paths  of  the  various 
planets  round  the  sun,  and  their  relative  distances, 
with  great  accuracy.  Kepler's  discovery  of  the  laws 
of  planetary  motion,  and  Newton's  of  gravitation 
followed  quickly.  The  invention  of  the  telescope  at 
once  led  to  the  recognition  of  the  fact  that  the  other 
planets  were  spherical  bodies  like  the  earth.  But 
still  the  scale  of  the  system  was  only  vaguely  known. 
The  distance  from  us  to  any  other  planet,  once 
determined,  would  give  all  the  information  required 
— the  other  distances  would  be  known  at  once.  Yet 
so  great  is  the  space  separating  us  from  our  nearest 
planetary  neighbour  that  only  within  the  last  century 
and  a  half  have  even  approximate  measurements 
been  attained,  while  not  more  than  thirty  years  have 
elapsed  since  measurements  worthy  to  be  called 
accurate  have  been  available.  Even  now  there 
remains  a  substantial  margin  of  uncertainty.  Thus, 
while  our  mean  distance  from  the  sun  is  certainly 
very  near  92,900,000  miles,  we  cannot  be  sure  it  may 
not  be  two  or  three  hundred  thousand  miles  more  or 
less.  In  other  words,  there  may  be  a  doubt  of  about 
one  half  per  cent  on  the  whole  distance.  Happily 
this  small  uncertainty  is  of  no  moment  from  our 


8 


GEOLOGY 


present  point  of  view.  We  now  know  the  scale  of 
the  Solar  System  with  ample  accuracy  to  allow  us 
to  view  the  earth  in  its  true  relations  to  the  other 
members.  It  will  materially  assist  our  subsequent 
discussion  if  we  here  append  a  table  giving  the 
relative  sizes  of  the  various  planets  and  their  distances 
from  the  sun. 

TABLE  OF  THE  SOLAR  SYSTEM.* 


Mean  distance 
from  Sun 
Miles 

Mass 
Earth=i 

Der 

Water=i 

sity 
Earth—  i 

SUN 

329,390 

1-40 

0-25 

MERCURY 

36,000,000 

0'055? 

5'56? 

i-oo? 

VENUS  .. 

67,200,000 

0'807 

5-14 

0-93 

EARTH  .. 

92,900,000 

rooo 

5-56 

i-oo 

MARS     .. 

141,600,000 

0-106 

3-92 

0-71 

MINOR  PLANETS 

JUPITER.. 

483,300,000 

314-50 

1'37 

0-25 

SATURN  .. 

886,200,000 

94'07 

0'64 

0-12 

URANUS.. 

1,782,800,000 

14-40 

1-35 

0-24 

NEPTUNE 

2,793,500,000 

16-72 

1-29 

0-23 

Two  large  planets  now  appear  in  the  system  which 
were  unknown  to  the  ancients,  being  very  remote 
and  consequently  too  faint  to  be  seen  without  a 
telescope.  Uranus  was  added  upwards  of  a  century 
ago  (1781)  by  Herschell,  Neptune  in  1846  as  one  of 
the  greatest  triumphs  of  mathematical  astronomy, 
its  existence  and  position  being  predicted  simultan- 
eously and  independently  by  Adams  and  Leverrier, 
from  a  study  of  the  disturbing  effects  of  its  attrac- 
tion on  the  movements  of  Uranus.  In  addition,  a 
host  of  "  minor  "  planets  swarm  between  the  orbits 
of  Mars  and  Jupiter.  They  already  number  near  six 
hundred,  and  year  by  year  more  fall  into  the  traps 
set  for  them. 

*  From  Ba.tt,<Spherical  Astronomy. 


THE  ORIGIN  OF  THE  EARTH  9 

Looking  over  the  preceding  table,  certain  facts 
stand  out  prominently.  All  the  bodies  in  the  system 
—even  the  sun  itself — are  minute  compared  with  the 
distances  between  them.  Should  we  construct  a 
model  in  which  the  sun  was  represented  by  a  2-foot 
globe,  the  earth  would  figure  as  a  pea  about  70  yards 
away.  Neptune  would  be  distant  nearly  a  mile  and 
a  quarter !  Again,  the  planets  fall  readily  into  two 
groups  as  regards  size;  near  the  sun  are  the  four 
small  planets,  among  which  we  have  to  number 
the  earth.  The  outer  planets  are  far  grander  in 
their  proportions.  In  the  same  model  which  has  a 
pea  to  represent  the  earth,  Jupiter  would  be  a  good- 
sized  orange.  Yet  all  the  planets  rolled  together 
would  be  quite  insignificant  compared  with  the  sun. 
As  to  the  six  hundred  minor  planets,  they  could  not 
build  up  the  earth  between  them. 

We  must  now  compare  the  earth  in  certain  other 
respects  with  its  neighbours.  Mars  and  Jupiter  will 
be  selected,  since  they  may  be  taken  as  typical  of 
the  rest,  and  are  much  better  known  than  the  others. 
Mars  is  a  globe  whose  diameter  is  a  little  more  than 
half  that  of  the  earth.  Viewed  with  a  first-class 
telescope  at  a  time  when  it  is  nearest  to  the  earth,  it 
presents  an  appearance  not  unlike  that  of  the  full 
moon  as  seen  with  the  naked  eye  or  with  an  opera- 
glass.  The  light  and  dark  areas  are,  of  course, 
different  in  form ;  they  have  perhaps  more  of  a  rude 
resemblance  to  the  form  of  the  continents  and  oceans 
on  the  earth.  And  land  and  water  they  have  long 
been  supposed  to  represent,  with  the  more  plausibility, 
in  that  the  darker  areas  have  a  greenish  tinge  while 
the  brighter  ones  are  distinctly  reddish,  giving  to  the 
planet  its  well-known  ruddy  light,  the  light  which  it 
reflects  from  the  sun.  It  is  not  possible  as  yet  either 


10  GEOLOGY 

to  confirm  or  deny  this  old  interpretation  of  the 
appearance,  though  recent  observations,  as  well  as 
certain  theoretical  considerations,  throw  considerable 
doubt  on  its  correctness.  It  may  be  well  to  recollect 
that  in  the  case  of  the  moon  the  dark  areas  were 
always  held  to  be  oceans  until  our  telescopes  clearly 
demonstrated  that  there  is  not  a  particle  of  water  on 
its.  surface.  On  Mars,  however,  we  observe  some- 
thing certainly  not  to  be  paralleled  on  the  moon. 
Over  each  of  its  poles  there  is  a  snow-white  cap, 
reminding  us  irresistibly  of  the  snowy  coverings 
around  the  terrestrial  poles.  In  this  matter  we  are 
able  to  make  an  observation  which  gives  us  greater 
confidence.  Mars  has  its  seasons  like  our  own,  and 
it  is  regularly  observed  that  as  each  Martian  pole  in 
turn  approaches  its  summer,  its  white  cap  diminishes 
in  area,  while  the  winter  is  marked  by  its  increase. 
Here  we  seem  to  have  a  clear  parallel  to  the  annual 
retreat  and  advance  of  our  own  polar  ice.  That  we 
are  really  dealing  with  ice  and  snow  seems  to  admit 
of  little  doubt.  It  has  been  suggested  that  the  ice 
may  not  be  frozen  water,  but  some  other  substance. 
There  is,  however,  no  evidence  that  such  is  the  case, 
and  it  is  not  probable. 

The  so-called  "  canals  "  of  Mars  are  so  familiar 
that  they  cannot  be  entirely  passed  over.  Let  us 
begin  by  saying  that  any  marking  on  that  planet  less 
than  ten  miles  in  width  would  almost  cetainly  be 
invisible  with  the  best  telescopes  under  the  finest 
conditions.  That  some  long,  straight  markings  exist 
on  its  surface  is  undoubted.  Whether  the  multitude 
of  finer  "  canals  "  which  make  up  the  wonderful  net- 
work reported  by  some  observers  have  any  real 
existence  cannot  yet  be  settled.  If  they  have,  they 
enable  us  to  draw  at  least  one  useful  conclusion,  for 


THE  ORIGIN  OF  THE  EARTH  11 

some  of  them  cross  the  "  seas  ."  Should  this  obser- 
vation be  confirmed  we  shall  have  destroyed  the 
oceans  of  Mars,  though  the  nature  of  the  "  canals" 
will  be  as  problematical  as  before. 

The  question  of  an  atmosphere  on  Mars  is  likewise 
involved  in  uncertainty.  It  seems  fairly  evident  that 
if  the  planet  were  supplied  with  air  as  abundant  and 
moist  as  that  round  the  earth,  it  would  make  itself 
more  conspicuous  than  it  does.  On  the  other  hand, 
some  observers  have  believed  that  they  have  observed 
slight  changes  of  appearance  to  be  attributed  to 
masses  of  cloud.  Certainly  slight  changes  do  occur 
in  the  aspect  of  the  planet  from  time  to  time,  which 
are  not  easily  interpreted  apart  from  an  atmosphere 
of  some  kind.  If  the  conclusion  favoured  by  the 
balance  of  evidence  is  correct,  that  the  atmosphere 
is  much  more  rarified  than  our  own,  then  that  is  a 
further  reason  for  doubting  the  existence  of  true 
seas  on  the  planet,  for  the  average  temperature 
would  probably  be  so  low  that  any  water  present 
would  normally  exist  as  ice. 

It  is  much  to  be  regretted  that  we  cannot  at 
present  speak  with  more  certainty  of  Mars,  for  it  is 
a  body  of  unusual  interest  from  our  present  point  of 
view.  But  let  us  mark  the  fact  that  so  far  as  the 
evidence  goes,  it  points  to  a  state  of  affairs  inter- 
mediate between  those  of  the  Earth  and  the  Moon. 
While  its  atmosphere  and  surface-water  are  probably 
less  abundant  than  on  the  Earth,  they  do  not  appear 
to  be  entirely  absent  as  in  the  case  of  the  Moon. 
Mars  clearly  shows  a  solid  surface  with  well-marked 
permanent  features,  by  the  observation  of  which  it 
may  be  seen  to  rotate  on  its  axis  in  a  period  only 
half  an  hour  longer  than  our  own  day. 

When  we  turn  to  Jupiter,  the  evidence  is  more 


12  GEOLOGY 

satisfactory,  in  spite  of  the  much  greater  distance  of 
that  body.  In  every  way  there  is  the  most  marked 
contrast  with  Mars ;  in  its  gigantic  size  —  its 
equatorial  diameter  being  eleven  times  that  of  the 
earth — in  its  great  polar  flattening,  but  most  of  all 
in  its  physical  condition.  The  most  moderate  tele- 
scope shows  the  familiar  dusky  belts  of  Jupiter, 
parallel  with  his  equator.  With  a  good  instrument 
the  picture  is  superb.  The  true  cloud-like  character 
of  the  bands  then  becomes  evident,  with  their  beauti- 
ful curling  contours  and  intertwinings  displayed  as 
in  a  most  delicately  toned  engraving.  And  on  Jupiter 
the  scene  is  always  new.  The  broad  outlines  of  the 
bands  may  last  for  months  or  years,  but  the  more 
delicate  cloud-tracery  is  in  constant  change.  No 
feature  on  the  surface  of  the  whole  planet  is  constant. 
We  see  merely  a  sea  of  clouds. 

Now  this  constant  and  universal  cloud-envelope  of 
Jupiter  is  more  remarkable  than  at  first  appears. 
True,  our  own  skies  are  commonly  enough  filled  with 
cloud,  and  were  our  atmosphere  more  dense  we 
might  never  see  the  sun  at  all.  But  our  clouds 
depend  for  their  existence  entirely  on  our  proximity 
to  the  sun,  whereby  the  surface  of  our  planet  is  kept 
at  a  temperature  which  causes  the  watery  vapours 
to  rise.  Jupiter  is  five  times  more  remote  from  that 
source  of  heat,  and  must  receive  a  correspondingly 
small  supply.  If,  therefore,  his  condition  resembled 
that  of  the  Earth  or  Mars,  any  water  on  his  surface 
ought  to  be  eternally  solid,  and  no  cloud  should  ever 
dim  his  skies.  Yet  we  rarely,  if  ever,  see  a  rift  in 
his  cloudy  shell.  Again,  all  the  winds  and  storms  of 
our  own  atmosphere  result  from  its  disturbance  by 
solar  heat.  Jupiter  should  be  comparatively  calm,  and 
still  most  violent  storms  occur.  Only  one  conclusion 


THE  ORIGIN  OF  THE  EARTH  13 

can  be  drawn ;  the  heat  which  is  not  supplied  from 
without  must  come  from  within :  Jupiter  must  be  a 
hot  planet.  Certain  other  observations  support  this 
conclusion.  The  markings  on  the  belts  enable  us  to 
mark  the  rotation,  and  to  make  the  noteworthy  dis- 
covery that  all  parts  of  the  planet  do  not  rotate  with 
the  same  period.  The  equatorial  regions  rotate  in  a 
shorter  time  than  those  nearer  the  poles.  Clearly 
the  atmosphere  of  Jupiter  does  not  form  a  thin  cover- 
ing over  a  solid  globe,  or  this  would  be  impossible. 
The  matter  below  the  visible  surface  must  be  gaseous 
to  a  great  depth,  if  not,  indeed  (as  is  not  improb- 
able) to  the  very  centre.  In  further  confirmation  we 
have  the  remarkable  fact  that  the  density  (i.e.,  the 
weight  relatively  to  the  size)  of  Jupiter  is  but  one 
quarter  of  that  of  the  earth.  He  is  but  little  heavier 
than  a  similar-sized  globe  of  water.  The  sun  has 
almost  exactly  the  same  density.  Either  this  means 
that  Jupiter  is  made  of  extraordinarily  light  materials 
(which  there  are  the  very  strongest  reasons  for 
doubting)  or  that  the  greater  part,  if  not  the  whole, 
of  the  great  globe  is  in  a  gaseous  condition,  which  is 
almost  tantamount  to  saying  that  it  is  at  a  high 
temperature. 

The  question  naturally  arises,  if  Jupiter  is  highly 
heated,  whether  he  should  not  be  self-luminous. 
That  would  obviously  be  a  matter  of  surface  temper- 
ature— of  the  visible  surface.  Direct  observation 
can  give  no  information  as  to  the  interior.  Even 
regarding  the  surface,  no  definite  answer  can  be 
given.  Undoubtedly  most  of  the  light  which  reaches 
us  from  that  surface  is  reflected  sunlight.  When 
one  of  the  four  larger  satellites  of  Jupiter  passes 
between  the  planet  and  the  sun  it  casts  a  shadow  on 
the  surface,  and  that  shadow  looks  intensely  black. 


14  GEOLOGY 

Yet  even  this  by  no  means  proves  that  the  surface 
is  not  luminous.  The  spots  on  the  surface  of  the 
sun  itself  look  as  intensely  black,  and  yet  are  certainly 
as  bright  as  an  electric  arc.  So  little  can  the  eye  be 
trusted.  On  the  other  hand  some  observers  have 
strenuously  maintained  that  signs  of  inherent  light 
are  visible  on  Jupiter.  However  that  may  be,  the 
evidence  previously  considered  gives  us  much  safer 
ground  for  judgment  on  the  physical  condition  of  the 
planet,  and  we  may  sum  it  up  with  little  hesitation 
by  saying  that,  as  Mars  shows  us  a  condition  inter- 
mediate between  that  of  the  earth  and  the  moon,  so 
Jupiter  divides  its  resemblances  between  the  earth 
and  the  sun. 

Little  more  can  be  gained  from  a  study  of  the 
other  planets.  Venus  and  Mercury,  as  far  as  we 
know  them,  are  closely  similar  to  the  Earth  and 
Mars  respectively — essentially  small,  cold,  heavy 
earth-like  planets.  Saturn,  Uranus,  and  Neptune 
are  light  giant  planets,  probably  like  Jupiter,  hot  and 
sun-like. 

To  complete  this  brief  survey  of  the  Solar  System 
we  have  to  enquire  what  is  known  of  the  physical 
condition  of  the  sun  itself.  The  vast  dimensions  of 
that  globe  are  difficult  to  realise,  its  diameter  being 
about  108  times  that  of  the  earth,  or  about  3i  times 
the  distance  which  separates  us  from  the  moon. 
Though  we  have  seen  it  is  relatively  only  one  quarter 
as  heavy  as  our  planet  its  actual  weight  is  330,000 
times  greater.  The  vastness  of  the  heat  and  light  it 
radiates  must  be  in  some  measure  evident  to  all. 
Though  the  problem  of  determining  the  real  tempera- 
ture of  its  surface  is  beset  with  very  great  difficulty, 
it  has  been  solved  with  sufficient  approximation  for 
our  purpose.  The  mere  statement  that  it  is  probably 


THE  ORIGIN  OF  THE  EARTH  15 

6000°  or  8000°  centigrade  conveys  little.  It  is  double 
the  temperature  of  the  electric  arc.  It  is  a  tempera- 
ture at  which  the  most  refractory  substances  we  know 
can  exist  only  in  a  state  of  vapour.  And  that  is  the 
temperature  of  the  surface  only ;  the  interior  must 
be  hotter  still. 

The  state  of  things  inferred  from  the  evidence  of 
temperature  is  confirmed  by  the  telescope.  The 
appearance  of  the  solar  surface  has  been  well  likened 
to  a  sheet  of  light  grey  cloth,  almost  hidden  under  a 
layer  of  freshly  fallen  snow.  The  "  snow-flakes  "  are 
dazzlingly  brilliant  clouds,  each  some  hundreds  of 
miles  across.  It  is  doubtful  if  these  are  true  clouds, 
consisting  of  liquid  droplets ;  if  they  are,  the  droplets 
are  of  iron  and  the  more  refractory  metals.  Possibly 
they  are  purely  gaseous.  Whatever  their  precise 
nature,  they  are  constantly  under  violent  agitation. 
Mighty  cyclones  sweep  them  about  with  perfectly 
incredible  velocities.  From  time  to  time  they  are 
completely  cleared  away  from  some  area  or  sucked 
down  towards  the  interior,  when  relatively  cool 
gases  from  above  fill  the  depression,  and,  cutting  off 
much  of  the  light  from  below,  create  a  false  impres- 
sion that  one  is  looking  through  a  hole  in  the  brilliant 
surface  to  a  dark  interior — a  mistake  to  be  carefully 
avoided.  These  "sun-spots"  are  among  the  most 
beautiful  objects  the  telescope  can  show.  When  we 
have  the  brilliant  disc  of  the  sun  hidden  behind  the 
moon  during  a  total  eclipse,  we  may  witness  another 
striking  manifestation  of  the  endless  turmoil  on  his 
surface.  Shot  out  in  every  direction  are  the  so-called 
red-flames,  in  reality  vast  jets  of  glowing  hydrogen 
and  calcium  vapour,  tens  or  hundreds  of  thousands 
of  miles  in  height.  Fortunately,  with  the  aid  of  the 
spectroscope,  we  are  now  able  to  cut  out  artificially 


16  GEOLOGY 

the  brilliant  light  of  the  disc,  which  ordinarily  over- 
powers the  light  from  these  "  flames,"  and  watch  them 
actually  thrown  out  from  the  surface,  rising  at  times 
with  the  stupendous  velocity  of  300  miles  a  second. 
The  most  violent  terrestrial  hurricane  is  perfect  calm 
by  comparison. 

In  its  rotation,  the  sun  displays  in  a  more  marked 
degree  the  peculiarity  we  observed  on  Jupiter:  the 
equatorial  regions  turn  more  quickly  than  the  polar 
ones.  While  a  point  on  the  equator  is  carried  com- 
pletely round  in  twenty-five  days,  the  period  of 
revolution  gradually  increases  to  upwards  of  thirty 
days  near  the  poles,  again  testifying  that  we  have  to 
do  with  a  gaseous,  not  a  solid  globe.  In  the  case  of 
the  Sun,  the  known  temperature  leaves  no  doubt  that 
it  is  gaseous  throughout.  But  perhaps  the  term  is 
misleading.  We  have  described  it  as  a  relatively 
light  globe ;  but  it  is  still  somewhat  heavier,  bulk  for 
bulk,  than  water.  And  that  is  merely  its  average 
density.  While  its  superficial  layers  must  be  much 
lighter,  its  density  must  gradually  increase  towards 
the  centre,  the  materials  at  any  depth  being  com- 
pressed under  the  weight  of  all  the  overlying  matter, 
so  that  the  central  portions  may  be  denser,  and  far 
more  rigid  than  steel.  That  the  Sun  as  a  whole  is  an 
intensely  rigid  body  is  amply  shown  by  the  absence 
of  any  perceptible  polar  flattening.  Such  conditions 
are  scarcely  consistent  with  our  ordinary  notions  of 
a  gas;  but  that  is  merely  because  we  are  only 
familiar  with  gases  under  low  pressures.  This 
general  density  and  rigidity,  nevertheless,  does  not 
prevent  a  very  evident  and  effective  circulation  of 
the  gases  from  the  interior  to  the  surface  and  back — 
an  important  fact  to  note,  since  it  must  keep  the 
materials  well  mixed,  if  we  may  so  phrase  it. 


THE  ORIGIN  OF  THE  EARTH  17 

We  can  learn  from  the  Sun  what  will  never  be 
possible  from  the  planets — the  nature  of  the  materials 
composing  it.  For  the  theory  of  that  wonderful 
weapon  of  modern  science,  the  spectroscope,  we 
must  reluctantly  refer  the  reader  to  some  work  on 
astronomy  or  physics.  Suffice  it  to  say  that  every 
substance,  when  in  a  hot  gaseous  condition,  and  not 
under  great  pressure,  shines  with  its  own  peculiar 
type  of  light :  strikes,  as  it  were,  its  own  character- 
istic notes  on  the  rainbow  scale  of  colour.  Every 
glowing  solid,  or  highly  compressed  gas,  on  the  con- 
trary, gives  out  the  entire  range  of  colours,  which 
blend  to  give  the  impression  of  white  light :  and  from 
such  light  any  ordinary  gas  will  pick  out  and  absorb 
the  colours  which  it  would  show  if  shining  itself, 
producing  gaps,  or  dark  lines,  across  the  rainbow 
band.  Precisely  this  condkion  we  have  in  the  sun ; 
the  light  from  the  dense  gases  of  his  brilliant  surface 
contains  all  the  spectral  colours.  But  as  it  leaves 
the  surface  it  has  to  run  the  gauntlet  through  the 
relatively  cool  gases  of  his  atmosphere.  Each  gas 
robs  it  of  certain-coloured  beams,  so  that  when  the 
light,  as  it  reaches  us,  is  analysed  by  the  spectro- 
scope, many  thousands  of  dark  lines  are  found  across 
the  rainbow-coloured  band,  representing  the  lost 
colours.  The  study  of  these  lines  results  in  a  dis- 
covery of  the  most  profound  significance:  of  the 
eighty  or  so  elements  which  the  chemist  has  recog- 
nised as  composing  the  multitude  of  substances  he 
has  been  able  to  analyse,  about  two-thirds  have 
thereby  been  found  to  be  constituents  of  the  Solar 
atmosphere.  Copper,  Zinc,  Iron,  Calcium  and 
Magnesium  are  a  few  of  the  gases  of  this  relatively 
cool  atmosphere  of  the  Sun.  No  more  striking  testi- 
mony could  be  found  to  the  intense  heat  of  that 

B 


18  GEOLOGY 

body.  But  of  far  greater  importance  is  the  indica- 
tion that  the  sun,  however  different  in  other  respects, 
is  composed  of  essentially  the  same  materials  as  the 
earth.  To  anyone  familiar  with  the  difficulties  of 
spectroscopic  investigation,  the  wonder  is  not  that 
we  should  not  yet  have  found  all  the  known  terres- 
trial elements  in  the  sun,  but  rather  that  we  should 
have  detected  so  many.  The  violent  circulation  of 
solar  gases,  already  referred  to,  assures  us  that  the 
composition  of  the  low-lying  layers  of  atmosphere  in 
which  the  absorption  of  light  chiefly  occurs  may  be 
taken  as  fairly  representative  of  that  of  the  sun  as  a 
whole.  And  with  that  remark  we  must  leave  the 
many  other  wonders  of  this  fascinating  globe  to  the 
astronomer. 

The  various  bodies  of  the  Solar  System  are  clearly 
very  fundamentally  related.  Though  we  can  never 
directly  know  the  material  of  which  the  other  planets 
are  made,  the  example  of  the  sun  confirms  what 
would  be  otherwise  probable — that  it  would  prove  to 
be  like  that  of  the  earth  itself.  And  many  likenesses 
pervade  the  system  besides  those  we  have  already 
noted.  All  the  planets  revolve  round  the  sun  in  the 
same  direction,  and  in  much  the  same  plane.  They 
all,  together  with  the  sun  himself,  rotate  on  their  axes 
again  in  the  same  direction.  We  cannot  but  believe 
that  the  history  of  each  must  be  intertwined  with  that 
of  the  others.  The  differences  among  them  are  differ- 
ences of  physical  condition.  The  Sun  is  intensely  hot, 
Jupiter  still  hot  enough  to  be  largely  or  wholly 
gaseous,  the  Earth  cold  and  solid  externally  but  with 
abundant  evidence  of  internal  heat,  Mars  certainly 
cold  on  its  surface,  while  the  Moon  shows  a  rugged 
exterior,  covered  with  the  scars  of  volcanic  fires  long 
since  extinct.  As  far  as  we  can  judge,  it  is  now  cold 


THE  ORIGIN  OF  THE  EARTH  19 

to  the  core.  The  present  state  of  the  various  bodies 
appears,  therefore,  to  depend  on  size. 

What  relationship  does  the  Solar  System  bear  to 
the  rest  of  the  universe  ?  So  soon  as  the  distance 
separating  us  from  the  Sun  became  known,  it  opened 
the  way  for  an  attack  on  the  larger  problem  of  the 
distance  of  the  stars,  and  we  are  now  in  a  position  to 
state  the  remoteness  of  some  of  the  nearer  ones 
with  tolerable  certainty.  But  miles,  or  even  millions 
of  miles,  are  quite  useless  as  units  in  which  to  express 
these  depths  of  space.  The  movement  of  light  pro- 
vides the  readiest  means  of  comparison.  A  light- 
beam  darts  through  space  with  a  velocity  of  over 
eleven  million  miles  per  minute.  A  little  more  than 
eight  minutes  suffices  for  it  to  bridge  the  gap  between 
us  and  the  sun ;  about  four  hours  and  a  quarter  will 
take  it  to  Neptune.  But  it  must  rush  on  through 
space  month  after  month  for  3\  years  before  it 
reaches  the  nearest  star,  and  it  is  likely  to  travel  as 
far  again  before  it  strikes  another.  Such  is  space. 
The  sun  himself  from  such  a  depth  must  appear  a 
star  of  small  importance  in  the  heavens. 

Clearly  the  stars  themselves  must  be  sun-like 
bodies.  Our  telescopes,  were  they  a  thousand  times 
more  powerful,  could  never  enable  us  to  see  them 
other  than  as  mere  points  of  light.  Yet,  knowing 
something  of  their  distances,  it  requires  no  elaborate 
calculation  to  show  that  many  a  star  must  give  out 
far  more  light  than  the  sun  himself.  And  while  the 
telescope  is  rendered  impotent  by  distance,  the 
spectroscope  can  still  analyse  the  light  of  the  star, 
and  learn  something  of  its  composition  and  physical 
condition.  The  result  is  to  add  enormously  to  the 
significance  of  what  was  learned  regarding  the  sun. 
Turn  where  we  may  throughout  the  universe,  the 


20  GEOLOGY 

same  familiar  substances  make  their  presence  known. 
Some  may  fail  to  appear  —  occasionally  nearly  all 
— but  we  look  in  vain  for  anything  entirely  novel. 
To  be  more  precise,  while  some  stars,  like  Capella, 
shine  with  a  light  which  can  scarcely  be  distinguished 
from  that  of  the  sun,  others,  like  Sirius  or  Vega, 
have  such  vast  atmospheres  of  hydrogen  that  the 
effects  of  other  substances  are  almost  masked. 
Others  again  seem  to  show  the  presence  of  certain 
chemical  compounds,  which  would  betoken  a  lower 
temperature,  while  we  must  add  to  the  list  those 
stars  which  no  longer  shine  at  all,  and  whose  exist- 
ence we  should  not  be  aware  of  but  for  their  proxim- 
ity to  other  stars  whose  movements  they  disturb. 
Some  of  these  differences  among  the  stars  appear  to 
be  attributable  directly  to  variations  in  size,  others 
are  certainly  due  to  different  temperatures. 

Whether  the  stars,  or  any  of  them,  have  attendant 
planets  we  can,  of  course,  never  directly  ascertain. 
Yet  the  study  of  their  probable  history  renders  it 
practically  certain  that  they  have.  Besides  the  stars, 
the  telescope  reveals  in  the  sky  large  numbers  of 
faint  cloud-like  bodies — the  nebulae.  They  appear 
of  every  size  from  masses  which,  despite  their  un- 
fathomable remoteness,  cover  areas  of  sky  larger 
than  the  moon  (invisible  to  the  naked  eye  because 
too  faint)  down  to  mere  hazy  specks  only  visible  with 
the  best  telescopes.  The  smallest  of  them  must  be 
larger  than  the  whole  Solar  System.  The  larger 
ones  must  occupy  the  most  inconceivable  regions  of 
space  ;  yet  so  flimsy  are  they  that  the  faintest  stars 
are  visible  through  all  but  their  densest  central  por- 
tions. The  smaller  are  usually  more  compact. 
They  pass,  indeed,  insensibly  into  nebulous  stars. 
What,  then,  are  these  airy  bodies,  and  what  are  their 


THE  ORIGIN  OF  THE  EARTH  21 

relations  to  the  stars  ?  Tested  again  by  the  quality 
of  their  light,  some  of  them  show,  like  the  stars,  the 
presence  of  many  of  our  familiar  elements  in  a 
gaseous  condition.  Others,  however,  show  nothing 
but  two  or  three  substances  which  are  gaseous  at 
ordinary  temperature  —  notably  hydrogen.  And 
these  nebulae  send  us  no  other  light  than  that  pro- 
duced by  the  glowing  of  these  few  gases.  But 
further  information  may  be  gleaned  from  certain 
bodies  which  partake  in  some  degree  of  the  nature 
of  nebulae  and  which  we  are  able  to  examine  at  much 
closer  quarters,-  namely,  the  comets  which  from  time 
to  time  appear.  Some  of  these  are  regular  members 
of  the  Solar  System,  others  visit  us  from  remote 
regions  of  space.  They  occasionally  pass  very  close 
to  one  of  the  planets,  or  even  collide.  Then  there  is 
a  disaster — to  the  comet.  The  space  within  the 
solar  system  is  literally  strewn  with  their  wreckage, 
and  the  Earth,  as  it  dashes  through,  attracts  the 
scattered  fragments  to  itself  in  millions.  When  a 
fragment  falls,  the  friction  created  by  its  headlong 
rush  through  our  atmosphere  raises  it  to  incande- 
scence, or  drives  it  off  in  vapour.  We  witness  the 
familiar  spectacle  of  a  shooting-star.  Of  all  the 
thousands  of  these  which  fall  on  the  earth  every  day, 
one  rarely  escapes  complete  dissolution  in  the  air. 
It  does  occasionally  happen,  however,  and  our 
museums  contain  not  a  few  of  these  justly  treasured 
meteorites — the  only  samples  of  the  outer  worlds  we 
can  ever  take  in  our  hands  to  examine. 

The  careful  examination  of  many  hundreds  of 
these  meteorites  has  not  resulted  in  the  recognition 
of  a  single  substance  with  which  we  were  not  already 
familiar.  And  there  is  every  reason  to  believe  that 
a  comet  is  nothing  more  or  less  than  a  swarm  of 


22  GEOLOGY 

myriads  of  these  bodies — the  dust  of  space.  The 
comet  glows  because  the  meteorites  clash  together 
as  the  swarm  rushes  on  its  course,  some  being  there- 
by made  hot,  others  possibly  vapourised,  though  the 
vast  majority  are  cold  and  black.  What  is  true  of 
the  comet,  is  in  all  probability  true  of  the  nebula. 
There  is  every  reason  to  believe  that  it  is  likewise 
an  immense  scattered  meteoric  swarm.  When  it  is 
very  widely  scattered,  collisions  among  the  particles 
are  few  and  the  general  temperature  consequently 
low.  Only  a  few  gases  therefore  shine  out.  But  it 
is  readily  demonstrated  on  purely  mathematical 
principles  that  such  a  vast  swarm  will  gradually  con- 
tract, in  consequence  of  the  mutual  attraction  of  all 
the  particles.  As  it  becomes  more  closely  packed, 
collisions  will  be  more  frequent,  and  the  general 
temperature  will  rise.  The  light  given  out  will 
gradually  become  more  like  that  of  a  star  until  the 
swarm  becomes  so  compacted  and  *he  temperature 
so  high  that  no  substance  can  longer  remain  solid, 
but  the  whole  becomes  a  globe  of  dense  intensely  hot 
gas.  It  has  ceased  to  be  a  nebula  and  become  a  star. 
Unable  to  contract  further,  its  accumulated  heat  will 
be  gradually  radiated  away,  the  successive  stages  of 
cooling  giving  rise  to  some  of  the  types  of  stars 
already  referred  to.  Cooling  will  continue  until  it 
rolls  through  space  a  dead  black  globe,  unless,  as  is 
very  probable,  some  passing  star  comes  so  close  as 
to  cause  its  disruption  and  a  return  to  its  primitive 
meteoric  condition,  when  the  whole  story  must  begin 
afresh. 

We  have  assumed  our  swarm  to  contract  abso- 
lutely uniformly  to  the  centre.  But  that  can  rarely 
happen.  Unless  it  is  uniformly  distributed  to  begin 
with,  minor  centres  of  condensation  must  be  set  up, 


THE  ORIGIN  OF  THE  EARTH  23 

and  if,  as  is  usually  the  case,  the  whole  nebula  has  a 
rotatory  motion,  these  minor  masses  will  never  join 
the  central  sun,  but  will  continue  to  revolve  round  it, 
while  each  contracts  and  collects  up  the  meteorites 
in  its  neighbourhood  as  it  gradually  resolves  itself 
into  a  planet.  Such,  in  its  broad  outlines,  we  believe 
to  have  been  the  origin  of  the  Earth.  Thus  in  the 
thousands  of  beautiful  spiral  nebulae  in  the  heavens 
we  may  watch  the  evolution  of  new  Solar  systems. 
In  the  central  condensations  we  see  the  future  suns ; 
in  the  knots  on  the  spiral  nebulous  folds  which  sur- 
round them,  the  embryos  of  their  planets.  And  in 
the  stars  and  the  greater  planets  we  may  witness 
something  of  the  stages  of  development  through 
which  the  Earth  itself  may  have  passed.  Precisely 
how  far  it  has  passed  through  them  it  will  be  the 
business  of  the  future  geologists  to  discover. 


24  GEOLOGY 


CHAPTER  II 

VOLCANOES  AND  EARTHQUAKES 

IN  the  preceding  chapter  we  have  considered  the 
Earth  in  relation  to  the  other  worlds  of  space,  and 
gathered  from  them  some  suggestions  as  to  its  early 
history.  It  now  becomes  our  duty  to  examine  a  little 
more  closely  the  Earth  itself,  and  to  see  what  manner 
of  world  it  is  on  which  we  dwell.  Our  direct 
acquaintance  with  it  is,  after  all,  very  feeble.  By 
united  effort  we  have  explored  most  of  its  surface, 
and  here  and  there  have  crept  a  mile  or  so  towards 
the  interior.  As  we  shall  find  in  the  sequel,  we  may 
even  examine  materials  which  have  been  perhaps 
ten  miles  deep.  But  what  of  that?  The  skin  of  an 
apple  bears  a  larger  proportion  to  the  whole  than  a 
ten-mile  layer  to  the  earth.  Two  questions  we  chiefly 
want  answered:  Is  the  material  which  forms  the 
visible  surface  of  the  globe  a  fair  sample  of  the 
whole  ?  And  what  is  the  physical  condition  of  the 
great  interior  mass  ? 

Indirect  evidence  alone  can  furnish  us  the  answer. 
Something  may  be  learned  from  the  weight  of  the 
globe,  and  something,  too,  from  its  behaviour  in 
relation  to  its  fellow-worlds ;  but,  most  directly,  the 
evidence  of  volcanoes  and  earthquakes,  with  their 
allied  phenomena,  comes  to  our  aid. 

The  problem  of  weighing  the  earth  is  a  highly 
interesting  and,  in  theory,  a  very  simple  one.  Every- 
one knows  that  the  weight  of  a  body  is  merely  the 


VOLCANOES  AND  EARTHQUAKES  25 

force  with  which  it  is  attracted  to  some  other  body — 
in  ordinary  cases,  to  the  Earth.  The  strength  of  the 
attraction — in  other  words,  the  weight — depends 
on  three  things :  the  amount  of  matter  in  the  body, 
the  amount  in  the  earth,  and  the  distance  between 
them  (or,  rather,  between  their  centres).  In  ordin- 
ary instances,  it  is  only  the  first  of  these  which  is 
varied.  Hence,  if  we  find  that  one  body  is  attracted 
with  twice  as  much  force  as  another  (i.e.,  if  it  is  twice 
as  heavy)  we  are  justified  in  concluding  that  the 
former  contains  twice  as  much  matter  as  the  latter. 
But  this  would  no  longer  be  true  if  we  varied  the 
other  factors.  For  example,  if  we  could  transport 
a  pound  of  sugar  to  the  surface  of  the  Sun,  we  should 
find  that  it  there  weighed  nearly  two  stones — but  there 
would  be  no  more  sugar.  The  increase  of  weight 
would  be  due  to  the  enormous  mass  of  the  Sun.  If 
these  simple  facts  are  comprehended,  there  is  no 
difficulty  in  understanding  how  the  earth  itself  may 
be  weighed.  Suppose  we  take  a  large  ball  of  lead, 
weighing  1  cwt,  and  place  near  it  a  small  ball  of 
some  kind.  Here  we  have,  in  miniature,  a  model  of 
the  earth  and  a  body  near  its  surface.  The  differ- 
ence is  one  of  scale  alone.  The  small  ball  is  attracted 
towards  the  large  one  (more  accurately,  of  course, 
they  attract  each  other).  The  attraction  is  all  but 
infinitesimal,  yet  with  sufficiently  delicate  apparatus  it 
is  measurable.  We  can  measure,  that  is,  the  weight 
of  the  small  ball,  due  to  the  attraction  of  the  large 
one.  But  we  also  know  its  weight  (its  ordinary 
weight)  due  to  the  attraction  of  the  Earth.  The 
difference  between  these  two  weights  is  clearly  due 
to  the  difference  between  the  amount  of  matter  in 
the  lead  ball,  in  the  one  case,  and  the  Earth  in  the 
other,  allowing  for  the  difference  in  their  respective 


26  GEOLOGY 

distances  from  the  smaller  ball,  also.  Hence,  know- 
ing the  amount  of  matter  in  the  lead  ball,  we  can 
calculate  by  proportion  the  amount  in  the  earth,  or, 
in  more  popular  language,  the  weight  of  the  earth. 
Various  methods  have  been  used  for  the  solution  of 
this  fascinating  and  important  problem,  but  the 
fundamental  principle  is  the  same  in  all — the  com- 
parison of  the  forces  with  which  some  small  body  is 
attracted  by  the  earth,  on  the  one  hand,  and  some 
other  body  whose  weight  is  known  on  the  other. 

The  statement  of  the  weight  of  the  earth  in  tons 
would  be,  of  course,  utterly  meaningless.  What  is 
significant  is  its  relative  weight,  which  may  best  be 
stated  as  almost  exactly  5i  times  that  of  an  equal- 
sized  globe  of  water,  or  more  than  two-thirds  that  of 
one  of  solid  iron.  As  a  whole,  therefore,  it  is  a  dis- 
tinctly heavy  globe.  This  knowledge  enables  us  to 
answer,  in  some  degree,  the  first  of  the  questions 
proposed;  for,  as  everyone  knows,  the  ordinary 
rocks  accessible  to  us  are  comparatively  light  sub- 
stances. They  vary,  naturally,  but  the  great  major- 
ity of  common  rocks  are  relatively  from  2j  to  3  times 
heavier  than  water;  and  we  may  say  with  perfect 
confidence  that  the  average  density  of  the  outer 
crust  of  the  earth  is  about  2|  times  that  of  water,  or 
almost  exactly  one  half  that  of  the  entire  globe. 
Whatever  the  reason,  it  is  clear  that  the  lighter 
materials  preponderate  on  the  surface,  and  the 
heavier  inside.  For  the  present,  we  will  be  content 
with  noting  the  fact. 

Volcanoes  form  by  far  the  most  impressive  testi- 
mony to  a  heated  condition  of  the  interior.  But  we 
must  note  in  the  first  place  a  much  more  general 
indication  of  the  same  fact.  In  these  days  of  deep 
mining,  everyone  is  familiar  with  the  fact  that  the 


VOLCANOES  AND   EARTHQUAKES  27 

rocks  become  hotter  as  we  descend  below  the  sur- 
face, this  being  one  of  the  most  serious  obstacles  to 
the  further  increase  in  the  depth  of  our  coal-mines. 
Every  deep  boring  or  sinking  which  has  been  made, 
in  every  part  of  the  world,  has  told  the  same  story 
of  a  more  or  less  uniform  rise  of  temperature  with 
increase  of  depth.  The  rate  of  increase,  or  tem- 
perature gradient,  as  it  is  called,  varies  considerably 
in  different  places,  so  that  even  a  fair  average 
cannot  easily  be  obtained.  The  most  generally 
accepted  figure  is  about  1°  Farenheit  for  every  60 
feet  of  descent.  From  our  present  standpoint  the 
exact  figure  is  of  no  moment.  Consider  what  it 
means.  The  figure  given  is  equal  to  a  rise  of  88 
degrees  to  the  mile  1  If  this  continues,  everything 
at  a  depth  of  a  few  miles  must  be  red-hot;  at 
twenty  miles,  the  temperature  will  be  above  the 
melting  point  of  the  most  refractory  substances 
known !  Here,  indeed,  is  an  astounding  conclusion. 
Can  we  consider  ourselves  justified  in  extending  the 
purport  of  our  observations  so  far? 

Corroborative  evidence  is  forthcoming  in  the  hot 
springs  found  in  every  part  of  the  world,  and 
especially  in  those  remarkable  springs — the  Geysers 
— from  which  super-heated  water  is  belched  forth ; 
but  most  of  all  the  volcanoes  support  the  conclusion. 

The  crude  notions  once  prevalent  concerning 
volcanoes  have  been  largely  cleared  away,  yet  few 
natural  phenomena,  perhaps,  are  more  vaguely 
apprehended.  Some  definite  conception  may  prob- 
ably best  be  attained  by  the  briefest  history  of  an 
actual  eruption.  Few  natural  disasters  in  modern 
times  have  compelled  more  sympathetic  attention 
than  the  devastation  of  Martinique  in  1902.  The 
West  Indies  are  encompassed  by  a  great  volcanic 


28  GEOLOGY 

zone,  but  the  island  of  Martinique  showed  no 
symptom  of  volcanic  activity  for  more  than  two 
hundred  years  after  its  discovery.  First  in  1792, 
violent  earthquakes  disturbed  its  unbroken  peace, 
and  a  slight  eruption  took  place  on  Mt.  Pele.  Quiet 
was  soon  regained,  and  lasted  until  1851,  during  the 
summer  of  which  year  a  sulphurous  odour  was 
observed  to  hang  round  the  mountain.  On  the  5th 
of  August  violent  explosions  were  heard,  and  a 
covering  of  dust  was  observed  over  the  summit  on 
the  following  day.  The  whole  disturbance  resulted 
from  explosions  of  gas  and  steam.  At  intervals 
until  the  end  of  the  year  these  outbreaks  were  con- 
tinued, after  which  quiescence  reigned  again  for 
nearly  another  half -century.  Already  in  1889,  how- 
ever, the  portents  of  coming  disaster  began  to 
appear.  Little  clouds  of  steam  began  to  curl  up 
from  the  mountain,  and  the  sulphurous  gases  re- 
appeared. In  1900  the  emissions  increased.  Earth- 
quakes became  frequent.  By  March,  1902,  the 
odour  of  the  gases  had  reached  St.  Pierre.  By  the 
22nd  of  April  the  earthquakes  were  so  severe  as  to 
break  the  submarine  cable  to  Guadeloupe.  Two 
days  later  the  column  of  vapour  and  dust  from  the 
explosions  rose  two  thousand  feet  above  the  moun- 
tain, and  the  next  day  began  to  fall  in  the  sur- 
rounding villages.  Explosion  followed  explosion 
every  two  or  three  minutes.  A  comparative  lull  on 
the  26th  was  but  the  calm  before  the  next,  and  in 
one  respect  most  frightful  storm.  Until  the  8th  of 
May  things  grew  steadily  worse.  Earthquakes 
became  most  frightful  and  almost  continuous. 
Ashes  began  to  fall  over  the  whole  island.  The  pall 
over  the  mountain  grew  in  size  and  denseness, 
putting  all  the  region  below  it  in  blackest  darkness. 


ERUPTION  OF  MT.  PELEE,  MARTINIQUE,  1902. 

Reproduced  by  the  kind  permission  of  Dr.  Tempest  Anderson. 

Upper  photograph  slightly  modified  by  retouching. 
Plate  II.]  [Geology,  28 


VOLCANOES  AND   EARTHQUAKES  29 

The  streams  burst  their  banks  from  the  torrents 
which  fell.  Lightning  and  thunder  in  the  cloud 
mingled  with  the  roar  of  explosion  and  earthquake. 
First  the  cable  to  Dominique,  then  that  to  St.  Lucie, 
broke  under  the  strain  of  repeated  shocks.  On  the 
fatal  8th,  the  great  cloud  rose  higher  than  ever 
before,  every  violence  was  at  its  worst,  when  the 
huge  curling  pillar  rolled  down  to  the  sea,  and  in  a 
few  minutes  blotted  out  St.  Pierre  with  its  twenty- 
eight  thousand  souls;  fire,  started  by  the  red-hot 
blocks  of  lava,  destroying  anything  which  was  not 
crushed  under  the  weight  of  falling  stone  and  dust. 
Blocks  of  stone  a  hundred  cubic  yards  in  size  fell 
mixed  with  smaller  stones  and  the  finest  dust. 
From  the  human  standpoint  the  eruption  had  done 
its  worst ;  but  it  yet  waxed  more  terrible.  The  20th 
and  26th  of  May  and  the  9th  of  June  were  each 
marked  by  accessions  of  violence  equal  to  that  of 
the  8th,  while  the  climax  was  not  reached  until  the 
30th  of  August,  when  all  previous  efforts  were  sur- 
passed by  the  production  of  a  cloud  several  miles  in 
height,  which  buried  half  the  island  in  dust  and  took 
toll  of  another  thousand  lives.  When  it  rolled  away 
the  island  looked  like  a  country  buried  in  snow. 

In  the  preceding  description,  reference  to  molten 
lava  has  been  purposely  omitted,  because  it  plays 
much  too  large  a  part  in  the  popular  idea  of  an 
eruption.  Lava  streams  were  ejected,  the  first 
being  seen  two  days  before  the  destruction  of  St. 
Pierre,  or  about  the  twelfth  day  of  the  outbreak. 
But  in  this,  as  in  most  eruptions  of  the  more  violent 
type,  the  great  mass  of  the  ejection  consisted  of 
solid  material  blown  out  and  shattered  by  the 
explosions,  much  of  it  reduced  to  the  most  impalp- 
able powder. 


30  GEOLOGY 

This  eruption  was  remarkably  similar  to  the 
classical  outburst  of  Vesuvius  in  the  year  79,  of 
which  we  have  so  circumstantial  an  account  in  the 
description  given  by  the  younger  Pliny  to  Tacitus. 
All  the  essential  features  were  exactly  repeated. 
One  sentence  of  the  narrative  deserves  quotation. 
Speaking  of  the  scene  presented  after  the  three 
days  of  perfect  darkness  we  are  told :  "  Every 
object  which  presented  itself  to  our  eyes — which 
were  extremely  weakened — seemed  changed,  being 
covered  over  with  white  ashes,  as  with  a  deep 
snow."  Everyone  knows  that  the  same  story  was 
repeated  again  in  the  recent  great  eruption  of 
1906. 

One  more  great  outburst  may  be  referred  to  by 
way  of  giving  a  more  complete  idea  of  the  magni- 
tude which  volcanic  forces  may  attain.  The  small 
island  of  Krakatoa,  in  the  strait  between  Java  and 
Sumatra,  showed  no  sign  of  volcanic  nature  undl 
1883.  In  May  of  that  year  a  comparatively  slight 
eruption  took  place,  but  only  in  the  following 
August  was  there  any  indication  that  a  violent 
disturbance  was  about  to  occur.  Then  in  two  days 
(the  26th  and  27th),  two-thirds  of  the  whole  island 
was  blown  away,  the  original  island  being  six  miles 
by  three.  New  islands  were  formed  in  the  neigh- 
bourhood, and  the  map  of  the  region  rendered 
completely  useless.  The  cloud  of  steam  and  ashes 
rose  to  a  height  of  17  miles,  and  darkness  extended 
for  150  miles  from  the  volcano.  The  fine  ash  was 
subsequently  carried  by  the  winds  to  every  part  of 
the  earth.  So  fine  was  some  of  the  powder  that  it 
was  estimated  that  about  twenty-thousand  particles 
would  go  to  a  single  grain.  Though  we  may  be 
thankful  indeed  that  the  event  occurred  in  a  sparsely 


VOLCANOES  AND   EARTHQUAKES  31 

populated  portion  of  the  globe,  no  less  than  36,380 
lives  were  lost.  The  sound  of  the  explosions  was 
heard  so  far  away  as  Rodriguez,  distant  2,500  miles, 
while  the  great  sea-waves  caused  by  the  accom- 
panying earthquakes  devastated  many  hundreds 
of  square  miles  of  country,  and  in  one  case  carried 
a  man-of-war  three  miles  inland,  and  left  it  stranded 
thirty  feet  above  the  sea.  The  foregoing  facts  are 


FIG.  1.— IDEAL  SECTION  OF  A  VOLCANO. 

Lava  black;  ash  dotted.    The  original  cone  has  been  partially 

destroyed,  and  a  new  one  constructed  within.     On  the  right, 

a  small  "parasitic"  cone  has  been  formed.     Dykes  of  lava  are 

seen  penetrating  the  various  rocks. 

indeed  a  mighty  tribute  to  the  forces  within  the 
earth.  And  they  are  no  mere  fancy  of  over-excited 
narrators,  but  the  result  of  evidence  collected  by  a 
special  commission  appointed  by  our  Royal  Society 
to  investigate  the  circumstances  of  the  eruption. 
Very  different  is  the  story  of  such  a  volcano  as 
Stromboli,  whose  constancy  has  well  deserved  its 
popular  appellation — the  "lighthouse  of  the  Medi- 
terranean." Year  in  and  year  out  the  puffs  of  steam 
lazily  curl  up  from  the  crater,  the  molten  lava  gently 


32  GEOLOGY 

heaves  and  falls  within,  carrying  with  it  the  slaggy 
floor,  and  rarely  rising  high  enough  to  cause  a  little 
to  well  over  the  side.  Violence  is  almost  unknown. 

It  needs  little  reflection  to  see  the  meaning  of  this 
difference  of  behaviour.  In  the  case  of  Stromboli, 
and  others  like  it,  the  volcanic  forces — the  steam 
and  other  gases — are  continually  escaping,  and  the 
energy  is  thereby  slowly  dissipated.  But  only  seal 
up  the  volcanic  pipe,  let  the  lavas  solidify  within, 
make  your  volcano  sleep,  and  then,  under  this 
outward  aspect  of  repose,  the  forces  must  gather 
within  till  they  burst  all  their  bonds,  and  a  great  and 
disastrous  eruption  is  the  result. 

Equally  evident  should  be  the  reason  why  a 
volcano  has  commonly  the  aspect  of  a  mountain  of 
more  or  less  comical  form.  Any  misconception  of 
this  point  may  be  removed  by  the  history  of  a  small 
hill  near  the  Bay  of  Baia?,  north  of  Vesuvius.  It 
bears  the  significant  name  of  Monte  Nuovo  (New 
Hill)  because  its  existence  dates  only  from  1538. 
Of  the  rise  of  this  hill  we  possess  the  account  of  an 
eye-witness.  Before  the  year  mentioned  the  site 
was  a  low-lying  tract  of  ground.  Volcanic  activity, 
as  usual,  was  announced  by  earthquakes  for  two 
years  before  the  event.  On  the  29th  of  September, 
1538,  twenty  shocks  occurred  in  the  one  day.  The 
following  night  a  slight  sinking  of  the  ground  took 
place,  followed  by  gushing  out  of  water,  first  cold, 
then  hot.  Later,  flames  were  seen,  and  then  the 
ground  was  torn  open,  and  an  eruption  of  ashes 
began,  which  in  two  days  built  up  the  present  hill, 
440  feet  in  height.  By  the  end  of  a  week  the 
eruption  ceased,  and  it  has  not  since  been  renewed. 
Here,  in  miniature,  we  have  the  history  of  all 
volcanoes,  their  cones  being  built  up  of  the  ashes 


VOLCANOES  AND   EARTHQUAKES  33 

and  lava  shot  out  from  the  vent,  whether  they  be 
small  hills  like  Monte  Nuovo,  or  vast  mountains  like 
Etna.  Yet  the  same  force  which  builds  up  the  cone 
not  infrequently  leads  to  its  destruction.  Many 
volcanoes  are  mere  wrecks  of  their  former  selves. 
The  old  Vesuvius — Monte  Somma — was  largely 
blown  away  by  the  historic  eruption  of  A.D.  79,  the 
present  cone  being  a  new  one,  while  in  such  a 
volcano  as  the  well-known  Kilauea  we  have  a  mere 
low  wall  left  round  a  great  lake  of  lava,  practically 
the  entire  cone  having  been  destroyed. 

The  realisation  of  the  true  nature  of  the  cone 
prepares  one  for  the  appeciation  of  a  fact,  the 
significance  of  which  may  otherwise  be  lost — the 
circumstance,  namely,  that  volcanoes  are  usually  to 
be  found  along  the  great  mountain  ranges  of  the 
world,  and  like  them,  therefore,  chiefly  by  the 
borders  of  the  great  oceans.  This  association  of 
volcanoes  with  mountains  is  readily  understood,  for, 
as  we  shall  see  in  the  sequel,  the  mountain  chains 
are  the  great  lines  of  weakness  of  the  earth's  crust, 
along  which  all  the  pent-up  energies  of  the  interior 
exhibit  their  most  profound  effects.  But  why  they 
should  be  found  bordering  the  oceans  is  a  problem 
much  more  profound  and  much  less  readily  solved. 
As  regards  the  volcanoes,  at  any  rate,  the  associa- 
tion has  given  rise  to  a  hypothesis,  which,  true  or 
not,  serves  to  emphasise  one  of  the  most  noteworthy 
of  volcanic  phenomena.  No  fact  stands  out  more 
prominently  in  the  accounts  which  have  been  given 
above  than  the  important  role  played  by  steam  in 
every  eruption.  It  is  the  motive  power  of  the 
entire  action.  The  explosions  which  hurl  solid  rock 
and  molten  lava  alike  into  the  air  are  almost 
entirely  explosions  of  steam.  The  earthquakes 

c 


34  GEOLOGY 

themselves,  which  are  so  inseparable  from  all 
violent  eruptions,  are  not  improbably  due  in  large 
measure  to  more  deep-seated  explosions  of  the 
same  character.  The  actual  amount  of  water  given 
off  in  the  form  of  vapour  from  a  great  volcano,  even 
in  a  brief  space  of  time,  baffles  imagination.  It  is 
not  without  reason,  therefore,  that  the  view  has 
long  been  put  forward  that  the  whole  existence  of 
volcanoes  is  due  to  water  which  has  percolated 
down  from  the  oceans  through  fissures  to  the  heated 
rock  below,  the  fluidity  of  which  has  been  increased 
by  its  solvent  powers,  till  the  mobile  mass  has 
yielded  under  the  pressure  of  the  overlying  rock 
and  sought  its  way  in  turn  through  some  fissure  to 
the  surface.  The  one  point  in  dispute  is  the 
ultimate  source  of  the  water.  But  whether  it  is 
derived,  as  described  above,  from  the  oceans,  or,  as 
others  suppose,  it  has  been  imprisoned  in  the  rocks 
since  the  earth  became  a  planet,  its  vital  importance 
is  undoubted. 

Nevertheless,  the  water  is,  of  course,  for  the  most 
part,  merely  the  agent  through  which  the  real  cause 
of  the  whole  phenomenon  is  made  manifest.  The 
internal  heat  of  the  earth  is  the  real  source  of 
energy  which  gives  the  water  its  explosive  power, 
which  shatters  the  rocks,  and  hurls  millions  of  tons 
of  them  miles  through  the  air.  Looked  at  in  this 
light,  the  vast  clouds  of  steam  and  solid  fragments, 
the  explosions  and  the  earthquakes,  are  no  less 
eloquent  of  the  vast  stores  of  internal  heat  than  the 
molten  rocks  themselves.  And  though  we  have 
referred  to  certain  limitations  in  the  distribution  of 
existing  volcanoes,  they  are  still  very  widely  spread 
over  the  face  of  the  globe.  The  entire  west  coast 
of  the  Americas  is  one  long  line  of  them  from  Alaska 


VOLCANOES  AND  EARTHQUAKES  35 

to  Cape  Horn.  Across  the  Pacific,  another  line 
stretches  from  the  north-east  of  Asia  through 
Japan,  the  East  Indies,  and  New  Zealand,  on  to  the 
South  Polar  Continent.  And  a  third  great  belt 
runs  across  the  whole  of  southern  Europe  and  Asia, 
practically  joining  the  other  two,  to  omit  all  refer- 
ence to  scattered  volcanoes.  These  are  merely  the 
volcanoes  of  the  present  day ;  were  we  to  include  all 
those  of  past  geological  ages,  scarcely  a  spot  on  the 
globe  would  be  free.  Little  wonder,  then,  that  the 
early  geologists  imagined  that  the  whole  earth, 
except  quite  a  few  miles  down  from  the  surface, 
must  be  one  molten  mass.  To  them,  the  "  crust "  of 
the  earth  was  a  perfectly  literal  crust. 

What  are  we  to  say  of  these  ideas  to-day  ?  The 
evidence  of  internal  heat  is  not  one  whit  less 
convincing  than  formerly.  Indeed,  the  more  ad- 
vanced state  of  physical  science  probably  justifies 
us  in  placing  more  reliance  on  the  general  indica- 
tions than  was  proper  for  the  older  geologists. 
There  seems  to  be  no  escape  from  the  conclusion 
indicated  at  the  outset,  that  at  a  depth  of  twenty  or 
thirty  miles  the  temperature  must  be  above  the 
melting  point  not  merely  of  the  ordinary  rocks  but 
of  every  substance  with  which  we  are  acquainted. 
Below  the  volcanoes,  as  we  shall  see  later,  reser- 
voirs of  molten  rock  exist  at  much  smaller  depths. 
Is  the  interior,  then,  liquid?  Perhaps  we  ought 
rather  to  ask,  is  it  gaseous  ?  For  most  substances 
remain  in  the  liquid  condition  through  a  compara- 
tively small  range  of  temperature;  after  they  are 
liquified,  they  are  soon  vapourised. 

Until  recently,  we  have  been  under  the  necessity 
of  appealing  again  to  the  astronomer  for  evidence 
on  this  question.  In  principle,  the  method  by  which 


36  GEOLOGY 

Sir  George  Darwin  has  attacked  the  problem  is 
readily  understood.  Everyone  knows  that  the  tides 
are  caused  by  the  attraction  of  the  Moon.  Owing 
to  that  body  being  comparatively  close  to  us  (its 
average  distance  is  rather  less  than  60  times  the 
earth's  own  diameter)  the  attraction  is  appreciably 
greater  on  that  side  of  the  earth  which  is,  for  the 
moment,  nearer  to  the  moon,  than  on  the  remoter 
side.  As  a  result  of  this  unequal  attraction,  the 
moon  constantly  tends  to  pull  the  earth  out  of 
shape;  and  in  the  case  of  the  oceans  it  succeeds, 
tides  being  the  result.  The  solid  globe  does  not 
appreciably  yield,  and  by  mathematical  analysis  it  is 
possible  to  calculate  what  its  rigidity  as  a  whole 
must  be  to  withstand  the  enormous  strain  which  the 
moon  puts  upon  it.  Darwin  concludes  that  it  must 
be  more  rigid  than  a  sphere  of  steel.  Everyone,  at 
least,  will  readily  understand  that  if  the  interior 
were  liquid  in  the  sense  once  supposed,  tides  like 
those  of  the  ocean  would  be  created  in  this  fluid 
mass,  which  the  feeble  solid  crust  would  be  utterly 
unable  to  withstand.  Without  paying  attention  to 
the  other  astronomical  methods  of  approaching  the 
problem,  let  us  turn  to  a  more  strictly  geological 
field  which  offers  great  promise. 

Earthquakes,  after  the  recent  disasters  of  San 
Francisco  and  Messina,  are  only  too  familiar  in 
their  general  manifestations.  The  heaving  of  the 
ground,  which  in  the  most  severe  shocks  may  even 
cause  the  earth  to  gape  open  with  the  strain,  the 
overthrowing  of  pillars,  chimneys,  and  even  houses, 
are  well-known  phenomena.  After  a  great  earth- 
quake, various  permanent  changes  are  usually 
observable  in  the  district  affected.  Streets,  railway 
lines,  and  similar  features  which  were  previously 


VOLCANOES  AND  EARTHQUAKES  37 

straight  have  become  more  or  less  crooked.  Fields 
which  were  rectangular  have  their  sides  no  longer 
square.  The  ground  in  some  areas  may  be  per- 
manently raised,  and  in  others  depressed.  In  some 
instances,  as  in  the  great  earthquake  which  affected 
the  Mino-Owari  districts  of  Japan  in  1891,  a  great 
line  of  fracture  may  be  developed  for  miles  across 
the  country,  along  which  it  may  be  raised  on  one 
side  or  depressed  on  the  other  to  the  extent  of 
even  twenty  or  thirty  feet.  Lateral  displacement 
of  the  ground  of  still  greater  magnitude  occurred 
along  the  line  of  fracture  in  the  San  Francisco 
earthquake.  These  displacements  are,  indeed,  the 
very  essence  of  the  earthquakes.  There  is  little 
doubt  that  the  majority  of  shocks  are  due  to  just 
such  sudden  slippings  of  the  earth's  crust  along 
some  line  of  fracture.  We  shall  see  later  that  these 
lines  of  fracture  and  movement  are  among  the 
commonest  of  geological  phenomena  in  every  part 
of  the  world ;  and  not  one  can  have  arisen  without 
giving  rise  to  an  earthquake.  Most,  indeed,  can 
only  have  resulted  from  a  long  succession  of  slips, 
each  giving  rise  to  a  more  or  less  severe  shock. 
The  difficult  question  of  the  ultimate  cause  of  these 
sudden  snappings  and  slidings  must  be  postponed 
awhile.  Meantime,  let  us  fix  our  attention  on  the 
nature  of  the  shock  itself.  Could  we  free  ourselves 
from  the  ground  during  an  earthquake  and  atten- 
tively watch  any  point  on  it,  we  should  see  it  moving 
up  and  down,  backwards  and  forwards,  side  to  side, 
in  a  most  complicated  dance.  But  it  is  just  the 
misfortune  that  we  cannot  free  ourselves  which 
makes  reliable  observation  difficult,  and  greatly 
exaggerated  statements  the  general  rule.  However, 
it  needs  no  refinement  of  observation  to  ascertain 


38  GEOLOGY 

the  essential  fact  that  the  earthquake  proper  is  a 
vibration  of  the  ground.  As  an  iron  bar  struck  on 
one  end  with  a  hammer  is  set  into  vibration  which 
travels  in  rapid  waves  along  the  bar,  so  the  earth, 
shaken  by  a  sudden  snap,  vibrates  in  like  manner, 
and  the  vibrations  spread  out  in  waves  in  every 
direction  from  the  place  of  the  shock.  The  waves 
travel  rapidly.  A  minute  suffices  for  them  to  cover 
several  hundred  miles ;  but  as  they  spread  out  they 
are  correspondingly  weakened,  so  that  they  soon 
cease  to  affect  our  senses.  The  study  of  these 
waves  is  the  essential  feature  of  the  science  of 
Seismology,  a  science  yet  in  its  earliest  infancy,  but 
one  which  is  already  shedding  light  on  the  state  of 
the  interior  of  our  globe,  and  which  seems  destined 
in  the  near  future  to  place  our  knowledge  of  that 
subject  on  a  surer  footing  than  it  has  ever  been 
before.  The  first  requisite  of  the  study  is  an 
accurate  record  of  the  movements  which  any  point 
of  the  ground  undergoes  during  a  shock.  Clearly 
this  can  only  be  obtained  by  referring  the  move- 
ments to  some  body  which  is  at  rest,  and  here  is  the 
problem:  how  to  obtain  a  " steady  point"?  No 
body,  of  course,  can  be  completely  detached  from 
the  ground ;  yet  we  may  have  it  detached  in  certain 
respects.  Take  an  ordinary  pendulum  for  example. 
If  the  support  to  which  it  is  attached  is  moved  up 
and  down,  the  whole  pendulum  must  be  carried  with 
it ;  but  if  the  support  is  moved  sideways,  the  weight 
of  the  pendulum  will  tend  to  remain  at  rest  where 
it  was.  If  now  the  support  remains  in  its  new 
position  the  pendulum  will  begin  to  swing.  If,  on 
the  other  hand,  it  moves  quickly  back  to  its  old 
place,  little  disturbance  of  the  weight  may  occur. 
Unfortunately,  no  matter  how  quickly  the  move- 


VOLCANOES  AND  EARTHQUAKES  39 

ments  are  performed,  some  swinging  will  occur; 
and,  moreover,  the  actual  movements  in  an  earth- 
quake are  comparatively  slow.  By  a  variety  of 
mechanical  contrivances,  however,  this  tendency  to 
swing  may  be  more  or  less  completely  counter- 
balanced, and  so  a  modified  pendulum  may  be  used 
to  record  the  horizontal  movements  of  the  ground 
during  an  earthquake,  a  pointer  attached  to  the 
pendulum  writing  on  a  recording  roll  of  paper  which 
moves  with  the  ground.  A  similar  pendulum  may 
be  constructed  which  swings  vertically  instead  of 
horizontally,  and  used  to  obtain  a  tracing  of  the 
vertical  movements. 

The  first  fact  we  learn  from  such  instruments — 
seismographas,  as  they  are  called — is  that  the  actual 
movements  in  an  ordinary  earthquake  are  small. 
Any  shock  in  which  the  ground  actually  rises  and 
falls  more  than  an  inch  is  a  severe  one.  In  the 
great  majority,  the  movement  is  much  smaller. 
We  are,  further,  able  to  confirm  another  important 
and  significant  observation.  An  earthquake,  in  the 
region  where  the  shock  originates,  is  not  heralded 
by  any  previous  trembling  of  the  ground.  The  full 
force  of  the  shock  falls  at  once,  without  warning, 
Now  this  is  not  the  case  when  a  shock  is  felt  at  a 
considerable  distance — say  one  or  two  thousand 
miles — from  the  point  of  its  origin.  In  such  a  case 
the  instrument  shows  a  gentle  trembling  for  some 
minutes  before,  quite  suddenly,  the  main  earth- 
waves  arrive.  At  such  a  distance,  of  course,  even 
the  principal  shock  is  scarcely,  if  at  all,  perceptible 
without  the  instrument.  Now  what  is  the  meaning 
of  these  "  preliminary  tremors  "  ?  Clearly  they  are 
vibrations  which  in  some  way  have  raced  the  main 
waves.  Those  latter  waves,  we  know,  travel  over 


40  GEOLOGY 

the  surface  of  the  earth,  much  as  common  ocean 
waves  travel  over  the  sea.  Can  it  be  that  the 
preliminary  tremors  are  waves  which  have  taken  a 
short  cut  through  the  globe?  Two  observations 
suffice  to  settle  this  point.  Firstly,  the  further  we 
go  from  the  centre  of  disturbance,  the  longer  is  the 
interval  Jby  which  the  main  shock  lags  behind  the 
preliminary  tremors ;  and  clearly,  the  further  we  go, 
the  greater  would  be  the  advantage  gained  by  waves 
cutting  through  the  earth.  Secondly,  when  we  time 
the  waves  accurately,  we  find  that  the  intervals 
taken  by  the  main  waves  to  pass  from  place  to  place 
are  proportional  to  the  distances  measured  over  the 
surface,  while  the  time  taken  by  the  preliminary 
tremors  to  reach  any  point  is  nearly  proportional  to 
its  distance  from  the  point  of  origin  measured 
through  the  earth. 

These  preliminary  tremors  now  possess  a  most 
profound  interest.  They  reach  us  after  having 
passed  through  the  innermost  recesses  of  the  globe, 
and  we  may  be  sure  that,  can  we  but  interpret  it 
aright,  they  bring  with  them  a  record  of  their 
adventures  within.  One,  and  that  the  most  im- 
portant, point  is  readily  gathered.  The  speed  with 
which  a  wave  travels  through  a  body  depends 
mainly  on  the  rigidity  of  that  body :  the  more  rigid, 
the  greater  the  speed.  Clearly  then,  if  the  interior 
of  the  globe  be  liquid,  or  diminished  in  rigidity  by 
the  intense  heat  towards  the  centre,  those  waves 
which  have  traversed  the  deeper  regions  will  be 
retarded.  What  do  we  find  ?  That  the  preliminary 
tremors  which  have  passed  most  deeply  through  the 
globe,  to  emerge  near  the  antipodes  of  the  point 
from  which  they  started,  have  travelled  a  little 
faster  than  those  whose  paths  have  not  been  so 


VOLCANOES  AND   EARTHQUAKES  41 

deep.  Here,  then,  we  have  a  striking  confirmation 
of  what  Darwin  has  told  us  from  his  studies  of  the 
tides.  The  interior  of  the  globe  is  at  least  as  rigid 
as  the  crust,  and  possibly  more  so. 

What  are  we  to  make  of  the  seeming  contradic- 
tion? On  the  one  hand  we  have  the  clearest 
evidence  of  high  temperatures  at  quite  small  depths, 
and  the  strong  probability  of  truly  sun-like  tempera- 
tures towards  the  centre.  On  the  other  we  have 
unimpeachable  testimony  to  the  fact  that  the  globe 
is  a  most  rigid  body  to  the  core.  If  you  consult  a 
text-book  of  geology  for  the  answer,  you  are  likely 
to  find  the  statement  that  the  interior  is  extremely 
hot,  but  is  kept  in  the  solid  condition  by  the 
enormous  pressure  of  the  overlying  portions.  It  is 
a  familiar  fact  that  when  a  body  is  kept  under  great 
pressure  its  melting  point  and  boiling  point  are 
usually  raised.  Unfortunately  the  physicist  is  likely 
to  point  out  that  there  is  a  limit  to  this  process,  that 
there  is  a  certain  definite  temperature  for  each 
substance — its  "  critical  temperature  " — above  which 
it  will  pass  into  the  gaseous  state  no  matter  how 
great  the  pressure  to  which  it  is  subjected;  and  he 
may  add  that  there  is  good  reason  to  believe  that 
the  temperature  within  the  earth  may  be  above 
the  critical  points  of  most  of  the  substances 
composing  it. 

Yet,  in  face  of  what  we  have  seen,  can  we 
believe  the  interior  to  be  gaseous  ?  We  must  leave 
the  responsibility  for  an  answer  with  the  physicist. 
Of  this  we  may  feel  certain,  that  before  the  heat 
may  compel  the  rocks  to  pass  into  a  gaseous  con- 
dition, the  pressure  will  be  so  enormous  that  a  gas 
under  it  will  have  no  resemblance  to  anything  of 
which  we  have  experimental  knowledge.  The  truth 


42  GEOLOGY 

is  that  we  have  to  do  with  a  state  of  things  al- 
together foreign  to  our  experience,  and  we  cannot 
postulate  what  the  properties  of  matter  may  be 
under  such  conditions.  Perchance  the  tremor  of 
the  earthquake  may  yet  enable  us  to  learn. 


CHAPTER    III 

THE     SOLID     ROCKS 

IT  is  now  time  to  interrogate  the  rocks  around  us. 
Some  of  them  readily  yield  their  story;  others 
divulge  their  secrets  only  after  patient  investigation. 
On  the  one  hand,  let  us  consider  the  rocks  which 
are  so  admirably  displayed  along  the  coast  from 
Whitby  to  Flamborough.  The  most  casual  glance 
at  those  magnificent  cliffs  suffices  to  show  that  the 
rocks  are  arranged  in  layers,  sheet  laid  over  sheet, 
through  thousands  of  feet  of  rock;  now  a  layer  of 
sandstone,  now  of  shale,  now  of  clay,  and  now  of 
limestone.  They  are  stratified  rocks  (compare 
Fig.  2).  We  take  a  piece  of  the  sandstone  and 
scrutinise  it  more  closely.  The  grains  of  which 
it  is  made  are  not  to  be  distinguished  from  the 
sand-grains  on  the  beach;  each  is  rounded  and 
smoothed,  as  those  are  being  made  in  the  dash  of 
the  waves.  We  crush  the  stone,  and  sand  it 
becomes.  Or  we  pick  a  fragment  of  shale  or  clay. 
While  dry  it  is  hard  and  compact ;  but  only  place  it 
in  water  and  it  crumbles  to  mud.  We  take  another 
block,  break  it  open,  and  find  within  the  beautiful 
form  of  a  coiled  shell  with  all  its  delicate  ornament ; 
but  not  the  form  merely — it  is  a  shell,  too  real  to 


THE  SOLID  ROCKS 


43 


44  GEOLOGY 

deceive  (see  Plate  IV.).  Fully  aroused  now,  we 
find  the  rocks  on  every  hand  filled  with  these  fossil 
remains  of  vanished  life ;  shells  of  every  form  and 
size,  corals,  sea-urchins,  fish  scales,  and  even  the 
bones  of  some  animal.  The  next  layer  of  rock  may 
yield  us  the  most  beautiful  fern  fronds,  or  fragments 
of  wood;  at  least  the  form  of  them,  for  the  sub- 
stance is  changed.  Or  we  chance  on  a  spot  at  the 
foot  of  the  cliff  where  the  storm  has  dislodged  a 
great  slab  of  sandstone,  and  see  the  surface  of  the 
bed  below  marked  with  ripples  as  perfect  as  those 
left  yonder  on  the  sands  by  the  retreating  tide. 

What  does  it  all  mean  ?  If  the  facts  around  you 
do  not  proclaim  the  answer  loud  enough,  move  on 
to  the  H umber,  and  there  watch  the  sandbanks  and 
mudbanks  growing,  layer  upon  layer,  out  of  the 
burden  brought  down  by  the  rivers,  burying  the 
cast-off  shells  left  upon  them,  and  only  awaiting 
consolidation  to  become  the  new  rocks  of  future 
ages.  Thus  readily  the  stratified  rocks  tell  their 
story,  and  show  that  where  is  now  dry  land  the 
ocean  rolled,  and  buried  on  its  sandy  bed  the 
remains  of  creatures  whose  very  kind  has  long  since 
passed  away.  They  are  built  up  out  of  the  wreck- 
age of  pre-existing  lands — for  sand  and  mud  result 
only  from  the  disintegration  of  the  rocks.  What 
then,  are  the  primitive  rocks,  if,  indeed,  any  may 
still  exist  which  have  not  been  through  nature's 
grinding  mill  ? 

Let  us  turn  to  one  of  the  granite  hills  of  the 
Scottish  highlands.  Here  is  no  trace  of  stratifica- 
tion, no  alternation  of  bed  with  bed,  no  sign  of 
fossils.  The  whole  mountain  is  one  uniform  mass 
of  granite,  whose  even  texture  is  reflected  in  the 
smooth,  monotonous  grandeur  of  the  hill.  We 


THE  SOLID  ROCKS  45 

come  closer  and  take  a  sample  of  the  rock.  No 
rounded  grains  betoken  the  rolling  of  the  sea. 
Instead  we  catch  the  glitter  of  crystals  whose 
polished  faces  have  never  yet  been  dimmed.  Still 
closer  scrutiny  reveals  the  fact  that  the  whole  rock 
is  crystalline.  Here  a  bright  metallic-looking  flake 
of  olive-brown  mica  is  embedded  in  a  fleshy-pink 
crystal  of  felspar.  There  an  irregular  space  is  filled 
with  clear  glassy  quartz.  Do  you  marvel  at  the 


A.  FIG.  3. 


A— Thin  slice  of  Granite  from  Dalbeattie,  Scotland,  magnified 
12  diameters.  The  clear  crystals  are  quartz,  the  cloudy 
ones  felspar,  those  with  numerous  parallel  cracks  mica. 

B — Thin  slice  of  Rhyolite  from  Elfdalen,  Sweden,  magnified 
12  diameters.  The  glassy  ground-mass  shows  "flow- 
structure,"  and  contains  multitudes  of  minute 
"  crystallites  "  which  cause  the  cloudy  appearance.  The 
large  crystals  are  much  broken  and  corroded. 

wonderful  accuracy  and  intricacy  with  which  the 
crystal  grains  are  fitted  together  and  interlocked? 
You  may  search  the  rock  with  lens  and  microscope ; 
not  the  smallest  space  is  left  unfilled.  The  truth 
proclaims  itself  irresistibly  that  the  minerals  have 
grown  together  (see  Fig.  SA).  How  can  such  a  struc- 
ture have  been  attained?  We  can  conceive  of  only 


46  GEOLOGY 

two  possible  answers.  Either  the  minerals  must  have 
crystallized  out  from  some  solution,  or  the  whole 
rock  may  have  been  in  a  molten  condition  and  have 
crystallized  as  it  slowly  cooled  and  solidified. 

It  is  indeed  not  easy  to  imagine  how  such  minerals 
as  felspar,  quartz,  and  mica  could  be  dissolved  in 
quantities  so  vast.  Water,  we  know,  has  wonderful 
powers  of  solution.  Even  such  minerals  as  these 
are  dissolved,  though  in  excessively  minute  propor- 
tions ;  and  in  the  early  days  of  geology  her  followers 
were  sometimes  less  disposed  to  bridle  the  imagina- 
tion with  the  sordid  considerations  of  mere  physical 
facts  than  this  matter-of-fact  age  demands.  More- 
over, there  was  ample  justification  for  the  belief 
that  the  primitive  oceans  of  the  young  earth  might 
have  been  highly  heated,  and  thereby  greatly 
enhanced  in  solvent  power.  So  arose,  during  the 
latter  half  of  the  eighteenth  century,  the  hypothesis 
that  rocks  of  the  granitic  class  represented  the 
earliest  deposits  from  the  primaeval  oceans,  a  view 
taught  by  Werner,  of  Freyberg,  and  spread  by  his 
pupils  over  the  length  and  breadth  of  Europe. 
When  the  waters  cooled,  they  were  no  longer  able 
to  hold  these  minerals  in  solution,  and  their  forma- 
tion ceased,  giving  place  to  the  deposits  of  mechanical 
sediment  which  form  the  rocks  of  later  ages.  But 
at  the  same  period  minds  were  not  wanting  more 
willing  to  abide  by  ascertained  physical  facts,  and  to 
observe  more  closely  the  phenomena  of  the  rocks. 
Our  countryman,  Hutton,  steadily  opposed  the 
Wernerian  teaching,  as  being  supported  neither  by 
chemical  and  physical  considerations,  nor  by  the 
facts  observable  in  the  field.  A  controversy  was 
carried  on  for  many  years  with  more  than  academic 
ardour  between  the  "  Neptunists,"  as  the  followers 


THE   SOLID  ROCKS  47 

of  Werner  were  dubbed,  and  the  "  Plutonists  "  who, 
with  Hutton,  believed  the  granitic  rocks  to  have 
resulted  from  the  slow  cooling  of  molten  material  in 
deep  reservoirs  below  the  surface — in  the  regions 
which  mythology  had  consecrated  to  Pluto.  Hutton, 
with  keen  insight,  perceived  a  crucial  test.  If 
Werner  were  right,  then  the  stratified  rocks  must 
lie  evenly  upon  those  of  the  granitic  class;  no 
confusion  could  occur  at  the  line  of  junction.  On 
the  contrary,  the  Plutonist  might  expect  to  find  his 
molten  granite  injected  into  all  the  cracks  and 
crevices  of  the  overlying  stratified  rocks.  It  was  a 
momentous  journey  when  Hutton  set  out  to  observe 
the  edge  of  the  granite  mass  in  Glen  Tilt,  and  we 
may  well  credit  the  story  that  his  exultation  was  so 
great  on  seeing  the  veins  and  strings  of  granite 
penetrating  into  the  adjacent  rocks  that  his  guides 
believed  he  had  discovered  gold.  For  him,  the 
great  controversy  was  ended,  and  he  had  won. 

The  reader  may  be  disposed  to  think  that  the 
controversy  was  unnecessary — to  say  that  volcanoes 
clearly  show  the  existence  of  reservoirs  of  molten 
rock  below  the  surface,  and  thereby  settle  the 
question  at  once.  But  not  so;  far  from  it.  The 
lavas  were  one  of  the  surest  strongholds  of  the 
Neptunists.  And  if  the  reader  will  take  a  piece  of 
lava  from  Vesuvius  in  one  hand,  and  a  fragment  of 
granite  in  the  other,  the  force  of  the  argument  will 
appear.  Indeed,  could  any  two  rocks  be  more 
unlike?  On  a  casual  inspection,  assuredly  not. 
The  latter  is  a  pale  grey  or  pink  glittering  crystalline 
rock,  the  former  a  nearly  black  dull  stony  mass. 
True,  a  closer  inspection  of  most  lavas  shows  a 
good  sprinkling  of  well-developed  crystals  scattered 
through  them,  but,  nevertheless,  the  ground-mass  of 


48  GEOLOGY 

the  rock  remains,  to  the  eye,  compact  and  stony. 
Hutton's  penetrating  sagacity  enabled  him  to  foresee 
the  explanation  of  this  striking  difference,  but  much 
patient  investigation  was  needed  before  his  views 
could  be  clearly  demonstrated. 

In  order  to  attack  the  difficulties  one  at  a  time, 
let  us  reject  the  Vesuvian  lava  for  the  moment,  and 
take  a  lighter  coloured  specimen  from  one  of  the 
volcanoes  of  the  Lipari  Islands.  Here  again  we  see 
the  stony-ground  mass  with  embedded  crystals, 
among  which  we  recognise  colourless  quartz  and 
felspar  and  blackish  glistening  micas.  To  make 
further  progress  we  must  appeal  to  the  microscope, 
and  examine  with  it  a  thin  transparent  slice  of  the 
rock.  The  picture  presented  as  we  look  down  the 
tube  reminds  us  strongly  of  the  surface  of  some 
sluggish  stream  covered  with  a  light  brown  scum 
which  winds  in  curling  eddies  round  floating  logs 
and  weeds  (see  Fig.  SB).  The  "  logs "  are  the 
crystals  we  have  already  seen,  the  "scum"  is  the 
ground-mass  of  the  rock.  But  the  eddies  are  no 
make-believe ;  they  are  frozen  solid,  but  they  carry 
us  back  irresistibly  to  the  day  when,  in  a  fiery 
stream,  the  rock  flowed  down  the  mountain-side. 
To  disclose  the  secret  of  the  ground-mass  we  must 
use  a  high  magnifying  power.  We  now  find  that 
this  owes  its  turbid  appearance  to  a  host  of  minute 
crystals,  some  just  recognisable  as  quartz  or  felspar, 
others  mere  rudiments  of  crystals  distinguishable 
only  as  minute  rods  or  beads ;  and  all  are  embedded 
in  a  structureless,  glassy  matrix,  which  is,  indeed,  a 
natural  glass. 

Experience  will  soon  teach  us  that  different  lavas 
vary  considerably  in  character.  The  large  crystals 
are  an  almost  constant  feature ;  but  in  the  stony 


THE  SOLID  ROCKS  49 

groundwork  we  discover  the  most  wonderful  variety. 
In  some  cases  we  find  nothing  but  pure  glass,  or 
glass  containing  only  swarms  of  the  little  rod-like 
and  bead-like  bodies;  in  others,  the  whole  is 
minutely  crystalline.  Nay,  the  same  lava  is  not 
uncommonly  glassy  on  the  edge  of  the  stream  and 
crystalline  inside.  And  here  we  have  a  little 
suggestion  which  may  guide  our  enquiry.  Everyone 
knows  that  if  we  aim  at  obtaining  good  crystals 
from  a  solution,  the  more  slowly  we  carry  out  the 
process  the  better.  Can  it  be  that  the  slowness  or 
rapidity  of  the  cooling  of  a  lava  has  something  to 
do  with  its  becoming  crystalline  or  glassy?  The 
observation  just  noted  suggests  that  it  may.  We 
may  experiment  by  completely  melting  up  a  lava 
and  cooling  it  artificially.  The  result  completely 
confirms  our  suspicion.  Only  by  the  slowest  possible 
cooling  can  we  get  any  crystallization  to  take  place, 
and,  at  best,  we  can  get  only  minute  crystals.  We 
have  solved  the  problem  of  the  varying  degree  of 
crystallisation  among  volcanic  rocks;  but  a  moment's 
reflection  will  show  that  we  have  raised  another. 
How  comes  it  that  the  large  crystals  occur  so 
profusely  among  them  ?  We  can  get  nothing  in  the 
remotest  degree  comparable  to  them  experimentally, 
though  we  may  reproduce  the  structure  of  the 
ground-mass  of  the  rock.  Is  it  that  the  great  mass 
of  lava  on  the  mountain  side  cools  so  slowly  that 
we  cannot  imitate  the  conditions  ?  If  so,  why  does 
not  the  whole  rock  consist  of  large  crystals  ?  But 
the  suggestion  is  altogether  crushed  if  we  examine 
one  of  the  fragments  of  lava  which  has  been  blown 
out  by  an  explosion  and  sent  in  a  molten  state 
hurtling  through  the  air — one  of  the  so-called 
volcanic  bombs.  This  must  have  cooled  and 


50  GEOLOGY 

solidified  in  a  few  minutes  merely;  yet  the  big 
crystals  appear  in  as  much  force  as  ever.  It  is 
clearly  out  of  the  question  that  they  could  have  been 
developed  in  so  short  a  space  of  time,  and  the 
conclusion  is  inevitable:  they  were  there  already 
when  the  stony  matrix  in  which  they  are  embedded 
was  still  in  a  molten  condition,  while  the  lava  was 
still  heaving  in  the  crater.  The  mind  is  now  carried 
back  to  the  deep  recesses  below  the  volcano  where 
for  untold  ages  the  molten  lava  has  been  hidden 
from  view.  We  see  it  now  in  imagination,  not  as  a 
wholly  liquid  mass,  but  with  slowly  growing  crystals 
floating  in  it.  How  slowly  the  cooling  and  crys- 
tallization must  proceed  under  the  vast  covering  of 
the  overlying  rocks  the  imagination  is  baffled  to 
conceive ;  but  of  this  we  may  be  sure — that  had  it 
been  allowed  to  continue,  each  crystal  would  have 
grown  until  no  drop  of  liquid  remained,  and  the 
whole  would  have  become  one  mass  of  coarsely 
crystalline  rock,  such  a  rock  precisely  as  granite  is. 

Here  we  have  the  explanation  which  Hutton 
already  foresaw.  In  their  deep  reservoirs,  lavas 
cool  with  excessive  slowness,  and  so  give  rise  to 
coarsely  crystalline  rock;  but  the  same  molten  mass, 
brought  to  the  surface,  will  solidify  with  compara- 
tive rapidity,  and  in  a  glassy  or  minutely  crystalline 
condition  which  gives  it  a  dull  and  stony  appearance. 
The  former  is  the  "  plutonic,"  the  latter  the 
"  volcanic  "  phase  of  the  same  rock. 

To  complete  the  enquiry,  the  British  Isles  will 
serve  us  better  than  Italy.  Britain  abounds  in 
volcanoes  on  every  hand.  We  do  not  recognise 
them,  because  even  the  youngest  of  them  have  been 
too  long  extinct,  and  their  cones  have  suffered  too 
much  from  exposure  through  countless  ages  to  the 


THE  SOLID  ROCKS  51 

elements.  But,  for  our  purpose,  this  is  so  much  to 
our  advantage.  The  cones  are  presented  for  our 
inspection  in  every  stage  of  decay,  dissected  for  us 
by  Nature  herself  so  that  we  may  examine  in  the 
light  of  day  every  detail  of  their  inner  structure. 
Here  we  have  exposed  to  view  the  old  lava  flow 
which  had  been  buried  for  ages  under  later  flows 
and  sheets  of  ash.  There  is  the  solid  plug  of  lava 
filling  the  old  pipe  through  which  it  was  rising  to 
the  crater.  Running  across  the  hillside  like  a 
ruined  stone  wall  stands  out  the  edge  of  a  vertical 


FIG.  4.— LION  ROCK,  ISLE  OF  CUMBRAE,  CLYDE. 

A  weathered-out  "  dyke  "  of  lava,  standing  as  a  wall-like 
mass  of  rock. 

sheet  of  lava,  which  had  been  driven  up  through 
some  rent  in  the  rocks  (see  Fig.  4).  Perhaps  the 
next  volcano  we  examine  has  had  its  entire  cone 
swept  away,  leaving  nothing  to  tell  of  its  former 
existence  but  the  old  "  neck "  of  lava  in  the  pipe, 
and  the  "  dykes  "  in  the  fissures  of  the  surrounding 
rocks.  In  other  cases  even  these  last  remnants 
have  been  removed,  and  the  deep  reservoirs  of 
lava,  now  frozen  into  granite,  exposed  to  view. 
We  have,  then,  every  opportunity  of  comparing 
lavas  which  have  undergone  their  final  cooling  in 


52  GEOLOGY 

every  possible  variety  of  position,  and  of  realising 
to  the  fullest  extent  the  large  number  of  distinct 
types  of  rock  which  are  produced  by  this  one 
varying  circumstance. 

The  typical  Vesuvian  lava  was  rejected  at  the 
beginning  of  this  discussion,  because  it  differs  from 
granite,  not  only  in  being  a  "  volcanic  "  instead  of  a 
"  plutonic  "  rock,  but  also  in  respect  of  the  minerals 
of  which  it  is  composed.  Quartz  is  absent.  The 
felspar  is  largely  replaced  by  beautiful  crystals  of 
colourless  leucite,  brownish  nepheline,  pale  blue 
sodalite,  or  bright  blue  haiiyne.  In  place  of  mica, 
augite  chiefly  occurs,  while  honey-yellow  olivine  is 
frequently  present  in  addition.  In  petrological 
language  these  facts  are  expressed  by  describing  the 
Vesuvian  lavas  as  basalts  (of  a  peculiar  type),  while 
the  volcanic  equivalents  of  granite  are  spoken  of  as 
rhyolites.  Basalts  likewise  have  their  plutonic 
equivalents,  of  which  perhaps  the  most  familiar 
British  examples  are  the  rocks  forming  the  rugged 
Cuilin  Hills  of  Skye.  These  latter  rocks  are  known 
to  the  petrologists  as  gabbros.  The  difference 
between  granite  and  rhyolite  on  the  one  hand,  and 
gabbro  and  basalt  on  the  other,  is  therefore  a 
difference  of  composition.  The  relative  proportions 
of  the  various  chemical  constituents  is  changed, 
with  the  result  that  when  the  liquid  mass  begins  to 
crystallize,  different  minerals  are  formed. 

The  reader  is  now  in  a  position  to  realise  how 
great  is  the  variety  of  rocks  of  this  character.  The 
composition  is  capable  of  almost  indefinite  variation, 
while  each  rock-type  so  produced  is  again  capable 
of  multiplication  according  to  the  conditions  under 
which  its  solidification  takes  place.  We  see  before 
us  an  immense  field  for  study,  which  considerations 


THE  SOLID  ROCKS  53 

of  space  forbid  us  now  to  enter.  Not  only  have  the 
innumerable  varieties  to  be  compared  and  classified, 
and  all  their  component  minerals  studied  individu- 
ally and  in  relation  to  one  another,  but  we  have  also 
to  discover  how  far  the  peculiarities  of  each  rock 
are  due  to  the  varying  conditions  of  pressure  and 
temperature  under  which  it  solidified.  We  have  to 
seek  for  the  laws  which  determine  what  minerals  shall 
be  formed  out  of  the  homogeneous  molten  magma 
from  which  the  whole  rock  arises.  And,  lastly,  we 
have  to  account  for  the  great  diversity  of  composi- 
tion which  is  found.  We  have,  in  fact,  to  discover 
the  complete  natural  history  of  igneous  rocks — for 
so  they  are  collectively  termed,  in  allusion  to  the 
"  fiery  "  state  through  which  they  have  all  passed. 

We  have  now  seen  what  are  the  two  fundamental 
types  of  rock  which  the  geologist  is  able  to  recognise 
— the  sedimentary  and  the  igneous.  In  other  words, 
we  arrive  at  the  generalisation,  after  hunting  the 
world  over,  that  every  rock  which  is  not  more  or 
less  clearly  built  up  of  the  worn  fragments  of  pre- 
existing rocks  has  attained  its  present  condition 
after  having  been'in  a  molten  state.  And,  since  this 
leaves  us  with  nothing  but  igneous  rocks  from  which 
ultimately  to  derive  the  materials  which  form  the 
sedimentaries,  we  finally  attain  the  still  broader 
generalisation  that  there  is  not  a  rock  on  the  earth's 
surface  whose  materials  have  not  at  some  time 
passed  through  the  molten  condition.  Here,  indeed, 
is  a  surprising  conclusion,  and  one  which  might 
have  seemed  incredible  but  for  the  known  facts 
which  have  been  detailed  in  the  preceding  chapter. 
Remembering,  however,  that  the  interior  of  our 
globe  is  certainly  in  a  highly  heated  condition  at 
the  present  time ;  seeing  that  the  sun  is  certainly, 


54  GEOLOGY 

and  the  larger  planets  are  probably  in  a  gaseous 
condition ;  noting  further  that  the  moon  shows  clear 
proof  of  its  former  possession  of  great  internal 
heat  which  has  now  vanished;  and  at  the  same 
time  recollecting  that  all  these  bodies  have  had 
their  histories  closely  involved  and  largely  common, 
the  conclusion  suggested  by  the  facts  now  to  hand 
is  clearly  that  the  entire  earth  was  formerly  in  a 
molten  state.  It  is  only  just  to  say  that  the  con- 
clusion is  perhaps  not  so  inevitable  as  might  at  first 
sight  appear,  because  the  action  of  the  elements  in 
destroying  the  rocks  is  so  powerful  that  it  is  in  the 
highest  degree  improbable  that  any  portion  of  the 
original  surface  of  the  globe  now  remains  in  its 
primitive  condition ;  and  owing  to  the  alternate 
exposure  of  rocks  once  deeply  covered,  and  the 
burial  of  other  areas  under  vast  accumulations  of 
sediment,  perhaps  no  rock  is  now  to  be  found  which 
has  not,  at  some  period  or  other,  been  more  or  less 
deeply  secluded  below  the  surface.  Yet,  notwith- 
standing that  these  considerations  compel  us  to 
admit  the  bare  possibility  of  an  alternative  explana- 
tion, the  supposition  that  the  now  solid  surface  was 
once  completely  molten  explains  so  simply  the 
cardinal  fact  indicated  above,  and  is  so  probable  on 
other  grounds,  that  little  doubt  need  remain  as  to 
its  truth. 


EARTH   SCULPTURE  55 


CHAPTER  IV 

EARTH    SCULPTURE 

WE  turn  now  to  a  new  aspect  of  our  subject.  In 
the  preceding  chapters  we  have  been  seeking  to 
gain  an  impression — necessarily  a  crude  one — of  the 
earth  as  it  may  be  supposed  to  have  resulted  by 
simple  cooling  from  its  original  nebulous  condition. 
It  has  not  been  possible  to  do  this  without  referring 
more  than  once  to  the  subsequent  changes  which 
have  been  brought  about  on  its  surface ;  the 
problems  of  geology  are  too  intricate  and  involved 
to  be  treated  independently.  Now  these  changes 
become  themselves  the  main  subject  of  enquiry. 

The  brief  span  of  a  human  lifetime  permits  us  to 
witness  very  little  change  in  the  scenes  around  us. 
Centuries  are  but  the  minutes  in  the  life  of  the 
earth.  Yet  even  this  short  time  suffices  to  effect 
much.  Here  the  storm  has  torn  out  a  new  gully 
on  the  slope  of  the  hill ;  there  the  river  is  ever 
undermining  its  bank  and  slowly  removing  the  field 
by  its  side.  The  ravages  of  the  sea  are  evident  to 
all.  If  one  stands  again  by  some  estuary  when  the 
tide  is  out,  the  scene  speaks  volumes.  There  are 
the  broad  banks  of  sand  and  mud,  surrounded  by 
the  brown  waters  of  the  river ;  out  to  sea,  the  water 
is  clear  and  blue,  the  mud  having  all  dropped  to 
swell  the  banks.  Even  more  striking  is  the  case 
when  a  river  flows  through  a  lake  in  its  course.  At 
the  upper  end  it  may  enter  as  a  turbid  stream ;  it 


56  GEOLOGY 

leaves  again  with  sparkling  water,  the  mud  sinking 
as  soon  as  the  current  is  checked  and  adding  to  the 
delta  which  forms  the  meadow  through  which  the 
river  enters  at  the  head  of  the  lake.  And  so  the 
lake  is  slowly  filled  till  meadow  alone  remains  to 
mark  its  site.  The  longer  one  contemplates  this 
burden  of  solid  matter  carried  by  every  river,  the 
more  one  becomes  impressed  with  the  enormous 
waste  which  the  land  in  the  course  of  ages  must 
suffer.  It  is  estimated,  for  example,  that  the  Danube 
carries  every  year  into  the  Black  Sea  enough  sand 
and  mud  to  make  a  sheet  of  rock  a  square  mile  in 
extent,  and  over  thirty-three  feet  thick,  or  about  67f 
million  tons  !  The  total  discharge  of  the  Mississippi 
may  be  eight  times  that  amount ! 

Everyone  is  aware  of  the  source  of  much  of  this 
material.  Every  shower  of  rain  produces  its 
thousand  tiny  streams,  each  busy  hurrying  the  finer 
particles  of  soil  to  the  nearest  brook.  Even  dry 
weather  cannot  prevent  the  waste.  Scorched  by 
the  sun,  the  soil  becomes  a  prey  to  the  wind  wher- 
ever protecting  vegetation  is  absent,  and  driving 
hither  and  thither  in  dusty  clouds  much  of  it  still 
finds  its  way  to  the  river,  and  so  to  the  sea.  It 
needs  little  imagination  to  see  that  if  the  loss  were 
not  in  some  way  made  good,  all  soil  must  ages  ago 
have  disappeared. 

Where  the  underlying  strata  consists  of  clay  or 
soft  sandy  rock,  nothing  beyond  the  moistening  by 
rain  and  the  penetration  by  the  roots  of  the  plants 
above  is  needed  to  convert  them  into  soil.  But 
what  of  .regions  where  underlying  rocks  are  hard 
and  compact  ?  No  one  can  have  failed  to  observe, 
in  quarry  or  road-cutting,  the  actual  condition  of 
affairs  in  such  a  case.  Below  is  the  solid  rock,  with 


EARTH   SCULPTURE  57 

only  a  barely  perceptible  joint  or  crack  here  and 
there.  Higher  up,  the  cracks  become  more 
numerous  and  obvious — the  rock  is  divided  into 
smaller  blocks.  Higher  still,  no  fair-sized  fragment 
of  sound  rock  is  to  be  seen  ;  it  is  completely  smashed 
into  small  angular  pieces,  most  of  which  have  been 
obviously  displaced.  Then  this  "subsoil"  passes  up 
imperceptibly  by  the  gradually  decreasing  size  of 
the  fragments,  and  discolouration  by  organic  matter 
into  the  true  soil.  Such  facts  speak  for  themselves. 
The  solid  rock  below  is  gradually  breaking  up  into  soil, 
and  making  good  the  waste  from  above.  But  how  ? 
It  is  a  matter  of  common  knowledge  how  the  roots 
of  trees  seek  their  way  into  the  smallest  cracks  and, 
growing  there,  exert  great  force  in  widening  the 
crevices.  This  must  clearly  help  in  the  process, 
yet  it  goes  on  equally  where  trees  are  absent.  The 
organic  acids,  again,  secreted  by  plants  into  the  soil, 
aid  in  the  later  stages  of  disintegration,  but  are  in 
no  wise  competent  to  begin  it.  Neither  will  the 
percolating  rain  explain  the  phenomena.  There 
still  remains  a  power  as  irresistible  as  it  is  gentle 
and  elusive — the  power  which  resides  in  the  rays  of 
the  sun.  As  the  rocks  are  gently  warmed,  they,  like 
all  other  solid  bodies  when  heated,  slightly  expand. 
On  cooling,  they  shrink  again.  The  daily  changes 
of  temperature  are,  of  course,  not  felt  through  more 
than  a  foot  or  two  of  soil,  but  the  influence  of  the 
seasonal  variations  is  felt  for  many  feet  down.  The 
result  is  clear  and  inevitable.  In  summer,  the 
temperature  of  the  superficial  portions  of  the  rock 
gradually  rises,  and  the  rock  expands,  while  the 
more  deep-seated  parts  are  unaffected.  The  effect 
is  that  with  which  everyone  is  familiar  when  a  piece 
of  glass  or  porcelain  is  unequally  heated — the 


58  GEOLOGY 

material  cracks.  Here,  then,  we  have  the  primary 
cause  of  the  numerous  cracks,  which  naturally 
become  more  numerous  towards  the  surface,  where 
the  temperature  variations  are  more  marked.  But 
the  story  does  not  end  here.  Later,  and  more 
indirectly,  the  sun  makes  its  influence  felt  still  more 
strongly.  As  winter  approaches,  the  rocks  shrink 
and  the  cracks  slightly  gape :  they  become  filled 
with  water,  and  then,  the  temperature  falling 
further,  the  water  freezes.  The  sudden  expansion 
of  water  when  converted  to  ice  is  familiar  to  all. 
The  cracks  are  thereby  widened,  only  to  be  filled  up 
again  with  water  on  the  next  thaw  and  the  process 
repeated ;  and  so  on  indefinitely.  Should  the  rock 
itself  be  at  all  porous,  the  freezing  of  water  in  its 
minute  pores  may  add  greatly  to  the  process  of 
gradual  disintegration.  Everyone  knows  how 
effective  is  a  frost  in  "  breaking  up  "  the  soil  itself ; 
and  it  is  but  the  same  process  on  a  greater  scale 
which  gives  rise  to  the  great  "tundras"  of  Northern 
Russia — firm  ground  when  frozen  in  winter,  but  vast 
death-traps  in  summer,  because  of  the  enormous 
depth  of  the  soft  soil  in  which  man  and  beast  may 
sink  out  of  sight. 

Thus  it  is  solar  heat,  acting  directly  in  the  alter- 
nate heating  and  cooling  of  the  rocks,  and  indirectly 
through  the  medium  of  water  which  freezes  when 
that  heat  is  partially  removed,  which  is  the  real 
cause  of  all  this  disintegration.  Yet,  so  far,  we  have 
been  considering  its  action  under  the  most  unfavour- 
able circumstances — in  the  lowlands,  namely,  where 
the  soil  is  largely  allowed  to  accumulate  and  so  to 
act  in  the  meantime  as  a  very  efficient  protection  to 
the  rocks  below,  screening  them  from  all  the  more 
violent  changes  of  temperature.  But  let  us  turn  to 


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EARTH  SCULPTURE  59 

the  hills.  On  the  mountain  top  no  soil  is  allowed  to 
gather;  and  those  who  have  been  in  these  lofty 
regions  will  have  no  difficulty  in  discovering  the 
reason.  There  the  battle  of  the  elements  proceeds 
with  a  vigour  unknown  in  the  lowlands.  The 
fiercest  lowland  gale  is  almost  a  daily  feature  of 
the  mountain.  The  rain  is  more  abundant,  and  is 
driven  with  almost  inconceivable  fury  against  the 
rocks.  The  gale  sweeps  before  it  not  merely  dust 
and  sand,  but  coarse  gravel  and  even  large  slabs  of 
rock  should  they  chance  to  present  a  wide  surface. 
Soil,  therefore,  can  never  gather.  The  bare  rock 
itself  is  exposed  to  the  fierce  heat  of  the  sun. 
Neither  need  we  dwell  on  the  greatly  increased 
power  of  the  sun  himself  at  these  altitudes.  The 
heavy  moisture-laden  air  below  has  not  yet  robbed 
the  solar  beams  of  their  power.  Hence,  as  every 
climber  knows,  under  the  mid-day  sun  the  rocks 
become  so  hot  as  to  be  almost  painful  to  touch ;  yet 
few  nights  pass  without  a  frost.  In  winter,  the 
bitter  cold  of  every  night,  coupled  with  the  abundant 
moisture  from  snows  melted  the  preceding  day, works 
still  greater  havoc.  If  one  stands  alone  in  the  awe- 
some solitude  among  the  mountain  crags  on  a  clear 
winter  day,  the  deathly  silence  is  broken  only  by  the 
sudden  crash  of  a  falling  block,  finally  dislodged 
from  its  place  by  the  last  night's  frost.  Crash  after 
crash  tells  us  we  are  listening  to  the  mountains 
falling  to  ruin.  But  our  eyes  reveal  even  more 
eloquent  testimony,  for  there,  on  every  hand,  are  the 
vast  piles  of  boulders  which  one  by  one  have  fallen 
from  the  peaks  above.  These  great  "  screes  "  are 
the  most  familiar  features  of  mountain  scenery ; 
every  gully  has  its  fan  of  boulders  at  the  bottom, 
every  cliff  has  its  long  boulder-slope  at  the  base. 


60  GEOLOGY 

Many  a  mountain,  indeed,  is  all  but  buried  under  its 
own  debris.  These  are  the  "  everlasting  hills  "  ! 
One  turns  away  with  a  deeper  sense  of  human 
littleness. 

In  our  own  islands  the  mountain  tops  are  largely 
protected  from  the  attack  of  the  winter  frosts  by 
their  mantle  of  snow.  Only  the  steepest  crags, 
where  the  snow  cannot  lodge,  are  constantly 
exposed;  and  there,  consequently,  destruction  is 
most  rapid.  The  same  partial  protection  is  afforded 
all  the  year  round  to  the  peaks  of  the  Alps,  whose 
glittering  mantle  never  completely  melts  away.  But 
there  the  snow  itself  turns  destructor.  Year  after 
year  the  snowfall  exceeds  the  quantity  melted,  and 
the  snow  tends  to  accumulate  in  ever-increasing 
mass  upon  the  hills.  On  the  steeper  slopes  its 
growing  weight  sooner  or  later  drags  it  down.  A 
mighty  mass  rushes  headlong  down  the  hill,  sweep- 
ing along  all  the  loose  boulders  in  its  path,  foaming 
through  the  forest  or  dragging  the  trees  down  in  its 
wild  career.  The  avalanche  is  no  small  instrument 
of  destruction.  But  far  more  effective  is  the  quiet 
and  almost  imperceptible  descent  of  the  snow  which 
is  left.  Growing  thicker  until  its  increasing  weight 
compresses  the  lower  layers  into  compact  blue  ice, 
we  then  witness  the  strange  fact  that  where  the  soft 
snow  held  firm,  the  solid  ice  begins  to  flow,  and  as  a 
glacier,  seeks  out  the  valleys,  and  creeps  slowly 
down  till  the  increasing  warmth  melts  it  away  as 
rapidly  as  it  advances.  Glaciers  are  the  rivers 
which  drain  the  snowfields  ;  and  the  flow  is  curiously 
alike  in  the  two  cases.  When  the  river  comes  to  a 
steeper  part  of  its  bed  it  breaks  into  a  cataract. 
The  ice,  under  like  circumstances,  becomes  cracked 
and  broken  up.  The  great  crevasses  gape  open,  to 


EARTH  SCULPTURE  61 

close  again  only  months  later  when  the  declivity  is 
passed.  Like  the  waters  of  the  river,  too,  the  ice 
carries  along  its  burden  of  wasted  rock.  From  the 
frowning  crags  by  its  sides  the  sun  and  frost  splinter 
off  blocks  which  are  hurled  down  on  to  the  sides  of 
the  ice  to  be  carried  along  in  bouldery  piles — the 
lateral  moraines — and  perhaps  to  be  tipped  off  in  a 
great  terminal  moraine  at  the  foot  of  the  glacier. 
But,  should  crevasses  open,  another  fate  may  await 
them.  Under  the  summer  sun  much  of  the  ice  on 
the  surface  of  the  blocks  left  between  the  crevasses 
may  melt,  and  the  boulders  may  slip  one  by  one  to 
the  bottom  of  the  yawning  chasms.  There  they  are 
firmly  gripped  by  the  icy  mass,  which  closes  over 
them  and  drags  them  forward  with  it  over  its  rocky 
bed,  crushing  them  down  on  to  it  at  the  same  time 
with  all  the  weight  of  the  ice  above.  Armed  thus 
with  thousands  of  engulfed  boulders,  the  glacier 
grinds  over  its  bed  like  a  mighty  rasp.  Rocks  and 
boulders  alike  are  ground  to  the  finest  powder,  and 
thus,  when  the  ice  finally  melts,  it  issues  from  the 
glacier  as  a  milky  stream,  carrying  with  it  the 
powdered  rock  from  its  bed.  Where  the  glacier  has 
retreated  from  its  former  channel,  the  rocks  are 
seen  smoothed  and  scored.  No  rough  crags  remain. 
In  their  places  are  the  most  wonderfully  smooth 
and  rounded  knolls  of  naked  rocks — the  roches 
moutonnees  (sheep-backs)  of  the  French.  And  by 
these  characteristic  effects,  as  well  as  in  other  ways, 
the  work  of  glaciers  may  be  recognised  in  many 
regions  where  they  are  not  now  to  be  found.  The 
smooth  outlines  which  are  so  characteristic  of  our 
own  hills  we  shall  later  find  reason  to  attribute 
largely  to  this  action. 
While  the  lower  mountain-regions  of  tropical  and 


62  GEOLOGY 

sub-tropical  countries  are  not  subject  to  erosion  by 
glaciers,  it  is  precisely  in  those  parts,  and  especially 
where  the  air  is  dry  and  rain  nearly  absent,  that  the 
action  of  solar  heat  is  most  pronounced.  In  the 
drier  regions  of  Africa,  mid-day  temperatures  of 
140°F.  are  no  uncommon  thing,  while  in  exposed 
places  the  thermometer  may  fall  a  complete  hundred 
degrees  in  the  night.  Exposed  to  extreme  changes 
such  as  these,  and  robbed  of  any  protective  covering 
of  vegetation  by  the  continual  drought,  the  rocks 
are  shattered  to  fragments.  Cracked  into  smaller 
and  smaller  pieces,  the  fragments  are  finally  caught 
up  by  the  gale  and  driven  along  till  they  are  at 
length  reduced  to  sand.  So  the  mountains  crumble 
down  till  they  are  surrounded  by  a  shifting  sea  of 
sand  formed  from  their  own  debris — a  typical 
desert. 

We  have  now  seen  some  of  the  principal  factors 
in  the  breaking  down  of  the  solid  rocks  of  the  earth's 
surface.  Watched  only  for  a  day,  their  effect  may 
seem  trifling.  In  a  lifetime  even  the  change  pro- 
duced may  be  relatively  small.  But  when  we 
remember  that  these  forces  have  been  ceaselessly 
active  not  only  for  thousands  of  years  but  for  many 
millions,  then  we  may  begin  to  realise  the  vastness 
of  the  changes  they  must  have  accomplished.  And 
we  shall  find  in  the  sequel  ample  evidence  of  changes 
vastly  greater  than  our  unaided  imaginations  would 
ever  have  been  likely  to  conceive. 

It  remains  to  ask  how  all  this  mass  of  debris  we 
have  seen  accumulating  on  the  land  is  ultimately 
transferred  to  its  burial-place  on  the  bed  of  the  sea. 
We  already  know  that  the  rain  and  the  rivers  are 
the  agents  of  transport.  But,  though  the  rain  may 
sweep  the  finer  soil  from  the  lowlands  into  the 


EARTH   SCULPTURE  63 

brooks,  it  is  powerless  to  move  the  boulders  on  the 
mountain  side.  It  does  ultimately  move  them,  not 
in  a  mass,  but  particle  by  particle.  Everyone  is 
aware  that  the  weather  attacks  all  exposed  stone, 
with  varying  rapidity  according  to  its  nature.  The 
picturesque  character  of  an  old  building  largely 
depends  on  the  removal  of  the  marks  of  artificial 
treatment  from  the  stone,  which  exposure  effects. 
Nowhere  can  the  effect  of  "  weathering  "  be  better 
observed  than  in  an  old  cemetery.  Nothing  could 
be  more  striking  than  the  difference  in  the  resistance 
to  decay  exhibited  by  the  various  stones,  whose 
inscribed  dates  at  once  tell  the  length  of  their 
exposure.  The  marble  stones  usually  crumble  most 
rapidly.  In  half  a  century  their  inscriptions  are 
likely  to  be  entirely  corroded  away  unless  protected. 
Granite  proves  itself  far  more  durable.  Its  polish 
may  dim  after  ten  or  twenty  years,  but  any  incised 
lettering  will  usually  be  quite  distinct  at  the  end  of 
a  century,  and  it  may  be  two  or  three  times  as  long 
before  it  is  completely  erased.  The  greatest  endur- 
ance, however,  is  exhibited  by  really  good  sandstone. 
Soft  sandstones  may  crumble  very  rapidly,  because 
they  are  porous  and  become  a  prey  thereby  to  the 
frost.  But  provided  that  it  is  really  compact  and 
hard,  the  centuries  pass  lightly  over  it.  One  or  two 
hundred  years  may  produce  scarcely  a  noticeable 
effect,  while  its  records  may  remain  legible  even  a 
thousand  years. 

The  most  familiar  observations  show  that  it  is 
exposure  to  rain  which  causes  this  decay.  Why  the 
great  differences  in  the  effect  produced  on  the 
different  stones  ?  In  their  general  physical  charac- 
ters the  three  rocks  considered  do  not  greatly  differ ; 
the  distinctions  are  mainly  chemical.  Marble  wholly 


64  GEOLOGY 

consists  of  crystals  of  carbonate  of  lime  ;  granite,  as 
we  know,  of  the  three  minerals  quartz,  felspar,  and 
mica;  sandstone  almost  entirely  of  grains  of  quartz. 
None  of  these  substances  are  greatly  affected  by 
pure  water ;  but  rain-water  is  not  pure.  It  contains 
dissolved  air.  The  addition  of  oxygen  and  nitrogen 
cannot  greatly  affect  its  powers,  but  the  small 
quantity  of  carbonic  acid  gas  in  the  air  makes  the 
water  acid — a  very  weak  acid,  it  is  true.  Now 
carbonate  of  lime  is  soluble  in  carbonic  acid ;  and, 
though  the  amount  of  this  acid  in  a  gallon  of  rain- 
water is  very  minute,  this  fact  is  compensated  for 
by  the  large  quantity  of  rain  which  in  a  few  years 
flows  over  the  stone.  Each  shower  removes  its 
mite  of  dissolved  material.  Even  so,  the  process 
would  be  very  slow,  but  for  the  fact  that  the  rain 
eats  in  between  the  tiny  crystals,  and  thereby  loosens 
them  so  that  they  may  be  washed  bodily  away.  If 
we  turn  now  to  a  long-exposed  granite  we  find  the 
surface  very  rough.  The  quartz  crystals  stand  out, 
while  the  felspars  are  all  more  or  less  eaten  away. 
In  time,  of  course,  the  particles  of  quartz  must 
tumble  out,  but  it  is  almost  entirely  to  the  corrosion 
of  the  felspar  that  the  gradual  decay  of  the  rock  is 
due.  And  so  the  sandstone,  consisting  entirely  of 
quartz,  might  endure  almost  for  ever  so  far  as  the 
chemical  corrosion  by  the  rain  is  concerned.  Most 
varieties  of  this  last  rock,  however,  are  very 
porous,  and  thereby  yield  readily  to  the  freezing 
of  water  in  their  pores,  or  the  solution  of  the 
cementing  material  which  holds  the  grains 
together.  The  great  importance  of  carbonic  acid 
in  the  final  disintegration  of  rocks  is  strikingly 
shown  in  the  much  more  speedy  decay  of  most 
building  stones  in  large  towns,  where  the  quantity 


EARTH   SCULPTURE  65 

of  this  gas  in  the  air  is  considerably  in  excess  of 
its  normal  amount. 

By  the  chemical  corrosion  of  the  rain,  then,  or  by 
the  continued  action  of  some  of  the  other  processes 
of  destruction  we  have  previously  considered,  the 
larger  debris  of  the  rocks  is  gradually  crumbled 
down,  and  swept  away  particle  by  particle  into  the 
rivers  and  thence  to  the  sea. 

Let  us  turn  now  to  the  rivers  themselves.  Among 
the  mountain  crags,  their  work  is  far  from  being 
confined  to  the  carrying  of  sand  and  mud.  Not  a 
little  of  the  bouldery  debris  from  the  peaks  above 
falls  more  or  less  directly  into  the  torrents.  Those 
who  have  never  stood  and  watched  the  mountain 
torrent  when  swollen  by  the  storm  can  have  little 
conception  of  the  power  of  the  waters.  In  fine 
weather,  only  the  gentle  splash  of  the  water  is  heard, 
but  in  spate,  when  the  volume  of  the  stream  is 
suddenly  increased  tenfold,  and  its  speed  is  corres- 
pondingly greater,  the  sound  of  the  water,  increased 
to  a  roar,  cannot  muffle  the  clash  of  the  pebbles  and 
boulders  as  they  are  rolled  over  or  dashed  along  the 
bed.  The  stream  is  converted  into  a  great  mill  in 
which  boulders  are  smashed  to  pebbles  and  ground 
to  sand.  While  large  angular  blocks  are  shot  in  at 
the  head  of  the  stream,  only  small  and  beautifully 
rounded  fragments  are  to  be  found  when  the  lower 
valley  is  reached.  But  not  only  are  the  boulders 
ground  on  one  another.  They  are  equally  ground 
on  the  rocky  bed  over  which  the  water  rushes. 
When  the  spate  is  over  and  the  stream  reduced  to 
its  normal  size,  the  beautiful  rounded  smoothness  of 
its  rocky  banks  testifies  to  the  grinding  they  have 
undergone.  So,  age  after  age,  the  torrent  runs  on, 
ever  cutting  deeper  and  deeper  into  the  rocks, 

B 


66  GEOLOGY 

carving  out  one  of  those  dark  winding  gorges  which 
are  the  most  exquisite  feature  of  highland  river 
scenery. 

In  the  middle  reaches  of  its  course  to  the  sea  only 
an  occasional  waterfall  revives  the  youthful  vigour 
of  the  stream.  Here,  leaping  over  the  edge,  the 
waters  dash  on  the  bed  below,  and  hurl  the  pebbles 
there  collected  again  and  again  at  the  foot  of  the 
rock  over  which  it  is  foaming.  The  incessant  battery 
slowly  undermines  the  fall  until  a  great  block  crashes 
down  from  the  top,  only  to  be  smashed  into  frag- 
ments  which  will  be  used  in  their  turn  to  continue 
the  constant  undermining.  So  the  fall  gradually 
retreats  up  the  river,  and  the  valley  is  slowly  cut 
deeper. 

The  final  course  through  the  true  lowland  regions 
has  its  quiet  flow  unbroken  even  by  these  episodes. 
Very  occasionally  an  unusually  severe  flood  may 
lead  to  the  sudden  carrying  away  of  large  quantities 
of  material  from  its  banks,  but  as  a  rule  the  stream 
meanders  quietly  through  its  meadows,  carrying 
with  it  a  certain  amount  of  mud,  and  sweeping  the 
sand  along  its  bed.  At  each  great  bend  the  current 
impinges  with  some  force  on  the  outer  bank,  which 
is  thereby  slowly  undermined  and  its  materials 
carried  away.  But  the  immediate  journey  of  this 
material  is  usually  short.  At  the  next  bend  in  the 
reverse  direction  the  waters  of  that  side  of  the  river 
are  on  the  inner  side  of  the  curve,  and  consequently 
become  "slack."  The  checking  of  the  current  at 
this  point  causes  most  of  the  material  which  has 
been  picked  up  from  the  bank  above  to  be  dropped 
here,  so  that  a  "  spit "  of  sand  and  gravel  grows  out 
at  this  point,  to  balance  the  loss  of  material  which 
the  stream  is  now  cutting  away  from  the  other  side. 


EARTH  SCULPTURE 


67 


And  so  at  each  bend,  while  the  outer  side  is  gradually 
cut  away,  the  inner  side  grows  by  the  deposit  of 
material  derived  from  the  erosion  at  the  bend  above. 
Reference  to  the  accompanying  diagram  should 
clear  away  any  difficulty  on  this  point.  A  little 
reflection  will  now  show  that  the  position  of  the 
river  channel  must  be  continually,  though  slowly, 
shifting ;  and  a  little  extra  patience  will  discover  that 


FIG.  5. 


FlO.  SA — Portion  of  course  of  meandering  river.  The  arrows 
indicate  the  course  of  the  main  current. 

FlO.  SB — Course  of  same  river  at  later  period,  changed  by 
erosion  of  banks  and  deposit  of  sediment.  The  areas 
over  which  the  meadow  has  been  worn  away  are  line- 
shaded.  The  growth  of  meadow  due  to  deposit  is 
indicated  by  dotting.  Note  that  the  curves  also  become 
constantly  exaggerated,  so  that  adjacent  bends,  by 
breach  or  the  narrow  isthmus  between,  may  join,  as 
would  shortly  take  place  at  the  lower  bend  here  shown. 


68  GEOLOGY 

the  ultimate  effect  is  of  the  nature  indicated  in  the 
next  diagram.  The  bends  of  the  river,  as  it  were, 
creep  bodily  down  the  valley.  Hence  the  deposit  of 
material  on  the  inner  sides  of  the  curves  is  only 
temporary.  As  the  next  bend  advances  down  the 
valley  it  is  picked  up  again  and  carried  on  another 
stage.  And  at  each  removal  more  grinding  takes 
place,  and  some  of  the  finer  material  each  time  will 
be  swept  by  the  current  right  on  to  the  sea. 

So  rivers  fulfil  their  great  function  of  transporting 
the  waste  of  the  rocks  to  the  sea,  while  performing 
themselves  not  a  little  work  in  the  way  of  earth- 
carving.  The  making  and  deepening  of  the  valleys 
belongs  entirely  to  the  rivers ;  the  widening  of  them 
is  the  work  of  heat  and  cold,  rain,  and  sometimes 
of  ice. 

Even  yet  we  have  not  exhausted  the  roll  of  forces 
which  conspire  to  destroy  the  dry  land  of  the  globe. 
The  attack  of  the  sea  is  so  familiar  that  it  is  un- 
necessary to  dwell  on  it  at  length.  Where  the  coast 
consists  of  soft  rocks,  the  toll  of  the  waves  may  be 
measured  by  feet  per  annum.  The  site  of  more  than 
one  old  town  on  the  East  Anglian  coast  now  lies 
below  the  sea.  In  Bridlington  Bay,  the  loss  is  said 
to  have  averaged  about  2i  yards  yearly  along  36 
miles  of  shore.  Yet  it  is  not  so  much  in  the  removal 
of  these  soft  materials  as  in  its  battles  with  the 
sterner  rocks  that  the  waves  show  their  power. 
Few  scenes  can  rival  in  grandeur  the  wild  rocky 
coast  in  a  storm.  When  the  roar  drowns  every  other 
sound  and  the  crash  seems  to  shake  the  very  founda- 
tion of  the  rocks,  wave  after  wave  retires  without 
appearing  to  have  made  the  slightest  impression. 
And  indeed  they  would  be  almost  powerless  against 
the  solid  phalanx  of  the  rocks  but  for  two  circum- 


EARTH  SCULPTURE  69 

stances.  In  the  first  place,  they  are  aided  by  the 
boulders  which  lie  at  the  foot  of  the  cliff.  These 
are  caught  up  and  flung  with  the  greatest  violence 
against  the  rocks,  till  their  foundations  are  cut  away 
and  block  after  block  falls  a  prey  to  the  waves.  But 
even  more  aid  is  derived  from  the  fact  that  the  rocks 
themselves  do  not  present  an  unbroken  face.  The 
joints  are  the  lines  of  weakness.  Into  these  crevices 
the  water  is  forced  with  all  the  pressure  of  the  wave. 
The  air  is  driven  in  and  compressed  before  it,  and 
when  the  wave  retires  it  expands  again  with  almost 
explosive  violence  and  drives  the  water  back.  When 
it  is  known  that  the  pressure  of  the  waves,  even  in 
the  North  Sea,  may  be  one  and  a  half  tons  to  the 
square  foot,  and  on  the  Atlantic  coast  twice  that 
amount,  one  is  in  some  position  to  realise  the  power 
this  action  represents.  The  joint  is  soon  widened  to 
a  cave,  and  the  cave  enlarged  till  its  roof  breaks 
down,  and  likely  enough  it  joins  with  an  adjacent 
cave.  So  the  sea  bores  into  the  cliff  all  along  the 
line,  and  gradually  cuts  back  the  coast.  In  this 
process,  large  pillars  of  cliff  are  frequently  isolated 
and  left  standing  out  in  the  sea  while  the  shore  re- 
treats behind  them — fit  monuments  to  mark  the 
conquests  of  the  waves.  These  outlying  "stacks" 
are  familiar  objects  all  round  our  more  rocky  shores, 
and  they  beautifully  illustrate  the  fact  that  the 
progress  of  the  sea,  no  matter  how  slow,  is  none  the 
less  sure. 

Here  we  must  conclude  this  very  brief  and  partial 
account  of  the  agents  which  carry  on  the  great 
process  of  rock-destruction  which  geologists  refer  to 
as  denudation.  When  we  reflect  on  the  number  of 
the  forces  which  combine  in  the  attack,  when  we 
remember  the  unceasing  activity  of  every  one  of 


70  GEOLOGY 

them ;  when  we  know,  too,  that  each  has  been  busy 
through  all  the  unnumbered  ages  since  the  earth 
became  a  planet,  then  we  may  well  wonder  how  it 
comes  that  any  land  still  remains  above  the  ocean. 
And  assuredly  it  would  not  be  so,  but  for  the  fact 
that  there  are  forces  counteracting  the  tendency  of 
denudation.  These,  however,  belong  to  another 
chapter. 


CHAPTER  V 

THE    SEA    FLOOR 

WE  have  now  traced  the  fate  of  the  materials  of 
the  wasted  rocks  as  far  as  the  sea.  It  is  time, 
therefore,  to  enquire  into  their  later  history;  and 
this  must  begin,  clearly,  with  an  investigation  into 
the  manner  in  which  they  are  deposited  on  the  sea 
floor.  The  very  extensive  dredging  and  sounding 
which  has  been  carried  out  during  the  last  forty 
years  over  the  whole  world,  and  especially  about  the 
great  highways  of  shipping,  has  put  us  in  possession 
of  an  ample  mass  of  the  facts  of  the  case,  The 
whole  bed  of  the  North  Sea,  for  example,  we  know 
to  be  strewn  with  shingle,  sand,  and  mud.  Here 
and  there,  of  course,  is  a  patch  of  bare  rock,  but 
these  are  not  sufficient  to  affect  the  truth  of  the 
general  statement.  The  same  is  true,  again,  of  the 
English  Channel,  the  Irish  Sea,  and,  indeed,  the 
whole  of  that  submerged  plateau  of  which  the 


THE  SEA  FLOOR  71 

British  Isles  are  only  the  hills.  For  an  elevation  of 
only  six  hundred  feet  would  drain  all  these  seas,  and 
cause  the  Atlantic  coast  to  run  from  Norway,  out- 
side the  Hebrides  and  Ireland,  down  to  the  Bay  of 
Biscay.  Beyond  this  line  the  existing  sea  rapidly 
deepens  into  the  great  trough  of  the  Atlantic.  In 
places  off  the  west  of  Ireland,  only  about  twenty-five 
miles  separates  points  where  the  depth  is  100  fathoms 
(600ft.)  and  1000  fathoms  respectively.  As  the  deeper 
waters  are  reached,  the  coarse  shingle  and  sand  are 
soon  left  behind;  the  sediments  become  finer  and 
finer,  until,  at  about  400  fathoms,  the  last  traces  of 
land-mud  are  found.  This  depth,  commonly  spoken 
of  as  the  mud-line,  marks  the  boundary  of  the 
terrigenous  deposits,  as  these  debris  of  the  land  are 
appropriately  termed.  It  may  be  a  matter  for  some 
surprise  that  the  distribution  of  these  deposits 
should  be  regulated  by  depth,  not  by  distance  from 
the  shore.  Where  the  waters  remain  shallow,  the 
land  sediments  may  be  carried  out  several  hundred 
miles;  on  a  rapidly  deepening  sea  bed  they  are 
limited  to  a  very  narnr::  belt.  But  let  us  consider 
how  they  come  to  be  distributed  at  all,  why  they  do 
not  wholly  accumulate  at  the  mouths  of  the  rivers 
or  the  foot  of  the  cliffs.  We  have  seen  that  when 
a  river  enters  a  lake,  its  sediment  is  dropped  as  soon 
as  the  water  is  brought  nearly  to  a  standstill.  So, 
again,  the  stoppage  of  the  current  on  entering  the 
sea  leads  to  the  deposit  of  material  at  the  mouth  of 
the  estuary.  The  power  of  water  to  carry  sediment 
depends  entirely  on  its  own  motion.  What  move- 
ment, then,  carries  the  sediments  about  in  the  seas? 
Everyone  is  aware  that  there  are  currents  in  the 
ocean,  the  most  familiar,  perhaps,  being  the  "  Gulf 
Stream  "  across  the  North  Atlantic,  These  are  set 


72  GEOLOGY 

up  mainly  by  the  unequal  heating  of  different  parts 
of  the  ocean  by  the  sun,  in  the  same  way  that  the 
irregular  heating  of  the  atmosphere  causes  the  winds. 
But  these  currents  are  much  too  feeble  to  carry 
sediment.  Around  the  coasts,  however,  very  much 
stronger  currents  run.  The  strongest  swimmer  may 
be  unable  to  make  headway  against  these,  and  they 
are  amply  strong  enough,  therefore,  to  sweep  before 
them  even  coarse  shingle.  These  are  tidal  currents> 
and  it  is  worth  a  moment's  consideration  to  under- 
stand how  they  arise.  The  tides  themselves  are  not 
currents;  they  are  waves,  in  the  open  ocean  only 
two  or  three  feet  high  but  several  thousand  miles 
broad.  Now  everyone  knows  that  in  an  ordinary 
wave  the  water  merely  moves  up  and  down.  Away 
from  the  shore,  a  boat  may  dance  up  and  down  on 
the  waves  all  day  without  being  moved  from  its 
original  position,  so  far  as  they  are  concerned.  The 
wave  travels  along,  but  not  the  water.  But  this  is 
no  longer  true  near  the  beach.  There  the  water 
rushes  with  great  force  along  with  the  wave.  The 
shallowness  of  the  water  in  the  latter  case  is  the 
cause  of  its  different  behaviour.  The  actual  up-and- 
down  movement  in  an  ordinary  wave  extends  down- 
wards in  the  water  only  a  very  few  feet,  and  so  long 
as  it  is  not  interfered  with  no  other  movement 
occurs.  But  as  the  shore  is  reached,  the  bottom 
does  interfere  with  it,  and  a  forward  and  backward 
rush  of  the  water  is  the  result.  So  with  the  great 
tidal  waves.  Even  at  depths  of  several  hundred 
fathoms  the  natural  rise  of  the  wave  entails  the 
bodily  movement  of  large  masses  of  water. 
Hence  we  see  that  these  tidal  currents  are 
necessarily  confined  to  the  comparatively  shallow 
waters,  As  the  waters  become  deeper  they  become 


THE  SEA  FLOOR  73 

weaker,  and  herein  we  have  the  explanation  of  why 
the  distribution  of  terrigenous  deposits  is  regulated 
by  depth  and  not  by  distance. 

Sand  and  mud  is  not  the  only  material,  however, 
which  the  tidal  currents  have  to  distribute  on  the 
sea  bed.  No  one  can  have  failed  to  observe  that  in 
places  the  shore  is  largely  or  entirely  made  of  sea 
shells.  Such  is  the  quantity  of  cast-off  shells  in  the 
sea  that  it  bears  no  inconsiderable  proportion  to  the 
sand  and  mud.  Here  and  there  whole  banks  of 
shells  or  patches  of  shell-mud  are  scattered  about. 
More  impressive  still  are  the  great  reefs  of  coral 
to  be  found  round  the  coasts  in  the  warmer  seas, 
where  the  exquisite  beauty  of  the  corals  themselves 
and  the  brilliant  colouring  of  the  profuse  sea-life 
among  them  adds  so  greatly  to  the  interest.  In  the 
Great  Barrier  Reef  of  Australia  alone  we  have  many 
thousands  of  square  miles  of  coral,  which  must  be 
at  least  hundreds  of  feet  in  thickness — it  may  be 
thousands.  Coral  and  shell  alike,  of  course,  consist 
of  carbonate  of  lime ;  nor  is  the  supply  of  this  sub- 
stance limited  to  their  productions.  Many  others 
among  the  more  lowly  inhabitants  of  the  sea,  besides 
shell-fish  and  coral-polyps,  make  themselves  limy 
coats  which  on  their  death  go  to  swell  the  calcareous 
deposits.  So  all  these  creatures  toil  to  form  the 
limestones  of  future  ages. 

Whence  comes  all  the  lime?  The  organisms 
themselves  obtain  it  from  the  water  of  the  sea,  in 
which  it  is  dissolved  in  small  quantity;  and  so  their 
ancestors  have  done  for  ages  before,  as  the  great 
masses  of  ancient  limestones  testify.  Yet  there  is 
no  evidence  that  the  creatures  of  to-day  have  any 
greater  difficulty  in  obtaining  a  supply  than  their 
early  progenitors.  The  explanation  lies  in  the  fact 


74  GEOLOGY 

that  the  supply  in  the  sea  is  restored  by  the  rivers. 
Rain-water  is  pure,  except  for  the  gases  it  has 
dissolved  out  of  the  air;  but  the  moment  it  reaches 
the  ground  it  begins  to  dissolve  the  rocks,  as  we 
have  already  seen.  Hence,  when  it  finally  flows 
back  in  the  rivers  to  the  sea,  it  bears  with  it  not 
only  the  rock  debris  in  suspension,  but  also  a  con- 
siderable amount  in  solution.  Among  this  latter 
material,  the  salts  lime,  soda,  magnesia,  and  potash 
usually  form  the  main  constituents.  To  the  igneous 
rocks,  then,  we  must  ultimately  trace  the  supply  of 
lime  which  forms  the  shells  of  the  sea  and  the  beds 
of  limestone. 

We  are  now  in  a  position  to  see,  in  broad  outline 
at  least,  the  relationship  of  the  various  sedimentary 
rocks  to  their  igneous  parentage.  Among  the  long 
and  varied  list  of  minerals  which  compose  those 
latter  rocks,  there  is  a  single  one  which  is  at  once 
abundant,  hard  enough  to  survive  the  eventful 
journey  from  mountain  to  sea  without  being  ground 
to  impalpable  powder,  and  incapable  of  decomposi- 
tion. Quartz  holds  this  record  alone ;  and  it  is  for 
this  reason  that  a  handful  of  sand  picked  up  at  the 
river  mouth  consists  almost  to  a  grain  of  quartz. 
The  most  abundant  of  all  the  minerals  of  the  igneous 
rocks — felspar — we  have  already  noted  as  yielding 
readily  to  the  corrosion  of  the  rain.  Its  dissolved 
portion  is  the  most  noteworthy  source  of  the  lime, 
soda,  and  potash  in  the  sea,  while  the  insoluble 
residue  is  washed  away  as  a  fine  mud  to  form  the 
future  clays.  The  remaining  minerals  for  the  most 
part  are  likewise  decomposed  or  ground  up,  and  so 
add  their  quota  to  the  salts  of  the  sea  or  the  muds 
on  its  bed.  Among  the  salts,  magnesia  and  iron  are 
their  principal  contributions. 


THE  SEA  FLOOR  75 

In  our  sandstones,  therefore,  we  get  back  the 
quartz  of  the  igneous  rocks;  the  shales  and  clays 
represent  the  insoluble  residues  of  their  other 
minerals ;  through  the  agency  of  the  organisms  of 
the  sea  the  dissolved  lime  is  extracted  from  its 
waters,  and  reappears  in  due  course  as  limestone. 
This  statement  is,  of  course,  incomplete,  but  it 
exhibits  in  a  few  words  the  main  features  of  the 
grand  process  of  the  reconstruction  of  rocks. 

The  remaining  dissolved  materials  are  slowly 
precipitated  from  the  waters  of  the  sea,  with  one 
exception.  The  magnesia  from  time  to  time  dis- 
places some  of  the  lime  from  the  limestone,  forming 
magnesian  limestone  or  dolomite.  The  iron  is 
deposited  among  the  sediments  as  their  great 
colouring  agent.  The  beautiful  red,  green  and 
yellow  tints  of  the  sedimentary  rocks  are  almost 
entirely  due  to  iron  in  various  states  of  combination 
with  oxygen  and  water.  Soda  alone  remains  dissolved 
in  large  quantities ;  owing  to  its  high  solubility  it  is 
not  deposited,  but  constantly  accumulates  in  the  sea, 
and  hence,  as  everyone  knows,  chloride  of  sodium  is 
now  the  salt  of  the  ocean. 

It  remains  for  us  to  consider  that  most  fascinating 
region  of  the  sea — the  floor  of  the  deep  ocean.  This 
was  an  unknown  world  until,  about  forty  years  ago, 
the  problem  of  deep-sea  sounding  began  to  be  seri- 
ously attacked.  Previously,  it  had  only  been  vaguely 
known  that  the  depth  of  the  great  oceans  is  to  be 
measured  in  miles.  Nothing  was  known  as  to  the 
real  conditions  at  those  depths,  or  as  to  the  state  of 
the  bottom.  It  had  been  confidently  predicted  that 
life  of  any  kind  must  be  impossible  from  the 
enormous  pressure  of  water  and  the  total  darkness. 
How  far  wide  of  the  mark  this  prediction  has  proved 


76  GEOLOGY 

is  now  familiar  knowledge.  It  is  difficult  to  resist 
the  temptation  to  stray  from  our  proper  path  to 
describe  the  wonderful  creatures  which  the  deep-sea 
dredge  has  brought  up  to  the  light  of  day,  to  remark 
on  their  extraordinary  forms  and  the  phosphorescent 
organs  which  serve  to  light  them  in  the  "  dark 
unfathomed  caves."  One  observation,  at  least,  we 
may  legitimately  make.  It  is  the  tendency  of  these 
deep-sea  creatures  to  develop  extraordinarily  long 
and  delicate  feelers  of  all  descriptions.  Some  of 
the  prawns,  for  example,  have  all  their  legs  many 
times  longer  than  the  body,  and  slender  to  a  degree. 
No  prettier  testimony  could  be  found  to  the 
unbroken  stillness  of  these  depths ;  such  creatures 
would  be  utterly  helpless  in  a  current. 

The  sediments  from  the  land,  as  we  have  seen, 
are  all  dropped  by  the  weakening  currents  by  the 
time  the  400  fathom  line  is  reached.  Beyond  this, 
the  dredge  in  most  places  brings  up  fine,  usually 
light-coloured  mud  or  "  ooze "  of  very  different 
nature.  On  examination,  fragments  of  the  delicate 
shells  of  some  of  the  shell-fish  which  pass  a  floating 
existence  on  the  high-seas  may  usually  be  found — 
sometimes  they  may  even  form  a  considerable  pro- 
portion of  the  material — but  usually  the  eye  recog- 
nises little  but  a  fine  powder.  How  different  when 
a  little  of  the  mud  is  placed  under  the  microscope ! 
The  grains  become  minute  shells.  Only  the  largest 
are  as  big  as  a  pin's  head,  yet  all  are  of  the  most 
exquisite  workmanship.  The  most  astonishing 
variety  of  form  prevails ;  shells  of  one  chamber 
and  of  many;  straight,  curved  or  coiled;  flat  or 
globular.  Some  are  chalky  and  opaque,  others  like 
crystal.  One  feature  characterises  them  all — they 
are  perforated  with  numerous  minute  pores  or 


THE  SEA  FLOOR  77 

foramina ;  whence  the  name  given  to  the  group  of 
creatures  whose  productions  they  are,  the  Fora- 
minifera.  But  all  the  shells,  we  observe,  are 
empty.  If  we  would  gather  them  with  their  living 
inhabitants  we  must  change  the  dredge  for  the 
tow-net,  and  drag  the  surface  waters  of  the  ocean. 
In  almost  every  part  of  the  world  they  abound. 
The  animals  themselves  are  mere  specks  of  almost 
formless  and  structureless  living  jelly,  and  one 
cannot  but  marvel  that  so  lowly  a  creature,  belong- 
ing to  the  lowliest  of  all  the  great  branches  of  the 
animal  world,  should  build  itself  so  beautiful  a  home. 
But  how  much  more  strange  does  it  seem  that  these 
most  weakly  of  builders  should  be  chosen  by  Nature 
to  provide  the  mantle  of  sediment  for  two-thirds  of 
the  earth's  surface — the  greater  part  of  its  ocean 
floor! 

Certain  noteworthy  differences  are  observable 
between  the  minute  shells  as  gathered  at  the 
surface,  and  as  dredged  from  the  greater  depths — 
say  from  2000  fathoms.  Those  from  the  surface 
have  all  their  ornament  sharply  defined;  they 
resemble  cut  glass.  They  are,  moreover,  in  many 
cases  provided  with  numerous  long  and  delicate 
spines,  which  help  to  keep  the  creature  afloat.  On 
the  other  hand,  the  shells  from  below  have  lost  the 
clear-cut  appearance,  and  their  spines  are  gone.  In 
view  of  the  great  stillness  of  the  waters  at  these 
depths  this  cannot  be  attributed  to  the  rolling  of 
the  shells  on  the  bottom.  It  must  be  referred  to 
the  corrosive  action  of  the  sea-water.  Besides  a 
small  quantity  of  carbonic  acid  dissolved  directly 
out  of  the  air,  the  sea  is  constantly  supplied  with 
this  substance  by  the  respiration  of  all  the  teeming 
life  it  contains.  In  consequence  it  is  able  to  dissolve 


78  GEOLOGY 

— very  slowly  indeed  —  carbonate  of  lime.  The 
delicate  shells  of  the  foraminifera,  while  occupied, 
are  protected  by  the  living  substance  which  sur- 
rounds as  well  as  fills  them ;  but  when  the  animal 
dies,  or  vacates  the  shell,  this  protection  is  lost,  and 
as  the  shell  slowly  sinks  to  the  bottom  it  is  exposed 
during  the  whole  time  to  corrosion.  The  minute 
size  of  the  object  causes  its  fall  to  be  very  slow — it 
may  be  only  a  few  yards  per  day — s6  that  as  much 
as  a  couple  of  years  may  be  occupied  in  the  journey. 
Ere  this  time,  many  of  the  more  delicate  shells  may 
be  completely  dissolved  ;  and,  clearly,  there  will  be  a 
limiting  depth  beyond  which  none  will  remain.  It  is 
found  at  nearly  3000  fathoms.  This  rain  of  foram- 
inifera on  to  the  bed  of  the  ocean  may  be  aptly 
compared  to  a  shower  of  snow  falling  through  warm 
air  to  uneven  land.  The  snow  reaches  the  high 
grounds  before  it  is  melted,  and  they  become  white 
with  the  fall,  but  in  the  valleys  the  snow  is  melted 
before  it  reaches  the  ground. 

At  least  one  great  deposit  of  foraminiferal  ooze 
was  familiar  long  before  the  depths  of  the  oceans, 
or  the  oceans  themselves  had  been  explored.  The 
white  cliffs  of  Albion  are  formed  of  just  such  a 
deposit  as  the  ooze  of  the  deep  Atlantic.  If  a  piece 
of  chalk  is  gently  crushed  and  the  finest  powder 
gently  washed  away,  there  will  remain  large 
quantities  of  foraminiferal  shells  almost  identical 
with  those  of  the  present  seas.  The  whole  mass  of 
the  chalk  is  almost  completely  composed  of  them  or 
their  broken  remains,  and  one  may  contemplate 
Beechy  Head  or  Flamborough  with  the  thought 
that  the  vast  thickness  of  chalk  which  is  there  par- 
tially exposed  to  view,  and  which  extended  over 
some  hundreds  of  thousands  of  square  miles  of 


THE  SEA  FLOOR  79 

Europe,  was  the  production  of  creatures  barely 
visible  to  the  eye. 

These  calcareous  oozes  of  the  ocean  bed  are  not 
without  admixture  of  inorganic  mineral  matter,  as 
well  as  some  organic  material  which  is  not  cal- 
careous. Those  wonderful  microscopic  plants,  the 
diatoms,  abound  in  some  parts  of  the  ocean,  and 
contribute  their  glassy  box-like  shells  which  they 
make  of  silica.  The  Radiolaria  are  equally  minute 
members  of  the  animal  kingdom,  somewhat  closely 
related  to  the  Foraminifera;  they  also  form  siliceous 
skeletons  of  the  most  delicate  lace-like  ^character 
which  in  places  form  almost  pure  siliceous  deposits. 
The  purely  mineral  matter  is  mostly  of  such  a 
nature  as  to  indicate  that  it  is  mostly  derived  from 
volcanic  dust  blown  over  the  oceans,  or  from  the 
decomposition  of  fragments  of  pumice  floating  over 
the  surface. 

In  the  deepest  basins  of  the  ocean  these  non- 
calcareous  deposits  alone  are  found.  The  proportion 
of  limy  matter  in  the  oozes  steadily  diminishes 
with  increasing  depth,  until,  as  we  have  seen, 
scarcely  any  remains  below  3000  fathoms.  A  fine 
red  clay  almost  universally  occurs  in  the  great 
deeps,  composed  of  such  materials  as  we  have  seen. 
How  slowly  this  red  clay  accumulates  is  strikingly 
shown  by  the  great  quantity  of  remains  which  lies 
unburied  on  its  surface.  Almost  every  haul  of  the 
dredge  brings  up  numbers  of  sharks'  teeth,  the  ear- 
bones  of  whales  and  the  like.  How  many  thousands 
of  years  must  it  have  taken  for  such  quantities  of 
these  objects  to  accumulate  1  Yet  so  slow  is  the 
formation  of  this  mud  that  they  lie  unburied  still. 
In  view  of  these  facts  it  is  indeed  not  improbable 
that  two  other  sources  of  supply  very  materially 


80  GEOLOGY 

contribute  to  the  growth  of  this  deposit.  When  the 
shells  and  skeletons  of  the  various  marine  creatures 
dissolve  there  will  be  in  most  cases  a  little  insoluble 
residue,  a  kind  of  "  ash  "  left  over,  which  may  still 
be  added  to  the  clays.  And,  lastly,  some  supply 
may  be  obtained  from  without  the  earth  altogether. 
The  shooting-stars  which  shower  down  on  to  the 
earth  in  thousands  every  day  probably  are  not  for 
the  most  part  larger  than  grains  of  sand,  and 
they  nearly  all  reach  the  surface  ultimately  as  fine 
dust;  yet  even  this  all  but  imperceptible  fall  of  dust 
may  add  appreciably  to  the  growth  of  the  red  clay. 
Thus  the  fineness  of  the  sediment  on  the  ocean  floor 
increases  regularly  from  the  gravel  on  the  shore  to 
the  impalpable  clay  in  the  deepest  hollows. 


MOUNTAIN  BUILDING  81 


CHAPTER   VI 

MOUNTAIN    BUILDING 

THE  last  stage  in  the  cycle  of  changes  through 
which  the  rocks  pass  yet  remains  to  be  considered. 
We  have  seen  the  primitive  igneous  rocks  crumbling 
under  the  action  of  the  sun  and  the  weather,  we 
have  watched  the  fragments  on  their  journey  to  the 
sea,  and  we  have  discovered  the  manner  of  their 
distribution  over  the  ocean  floor.  We  know,  further, 
that  the  deposits  of  former  days  have  been  elevated 
above  the  sea,  so  that  our  existing  lands  consist  very 
largely  of  rocks  of  this  sedimentary  character.  This 
elevation  completes  the  cycle ;  and  a  cycle  it  really 
is,  for  no  sooner  do  the  new  deposits  peer  above  the 
waves  than  they  become  a  prey  again  to  the  agents 
of  destruction,  so  that  the  whole  series  of  events 
may  begin  anew. 

The  mere  existence  of  sedimentary  rocks  (with 
often  the  old  sea-shells  in  them)  now  high  among 
the  hills,  is  proof  enough  that  the  sea-level  did  not 
always  have  the  same  relation  to  those  lands  that  it 
has  to-day.  But  the  observation  gives  no  indication 
whether  it  is  the  land  that  has  risen  or  the  sea  which 
has  subsided.  It  may  seem  more  natural  to  con- 
clude that  the  sea  has  changed  its  level,  and  that 
view  seems  to  have  been  adopted  by  the  ancients. 
A  fuller  consideration  of  the  facts,  however,  leads  to 
the  opposite  conclusion.  Let  us  note,  firstly,  that 
the  positions  of  the  sedimentary  rocks  are  by  no 
means  the  only  indications  of  change  of  level.  Few 


82  GEOLOGY 

persons  can  have  failed  to  notice  that  many  of  our 
seaside  towns  are  built  on  remarkably  flat,  low-lying 
stretches  of  ground,  while  the  country  immediately 
behind  is  frequently  hilly,  the  hills  rising  abruptly 
from  the  flat.  The  North  Wales  coast  furnishes 
perhaps  the  most  striking  examples.  Almost  every- 
where, however,  these  low-lying  terraces  are  to  be 
found ;  here,  it  may  be,  only  a  few  yards  wide,  there 
several  miles.  Not  uncommonly,  especially  round 
the  estuaries  of  the  Forth  and  Tay,  behind  the  first 
low  terrace  may  be  found  a  second  at  a  higher  level, 
with  an  abrupt  step  from  one  to  the  other.  There 
may  even  be  several  of  these  steps  one  above 
another.  A  little  investigation  soon  reveals  the 
meaning  of  these  terraces.  Where  the  rocks  are 
hard,  the  terrace  is  commonly  backed  by  regular 
cliffs,  on  which  the  marks  of  the  sea  are  not  to  be 
mistaken.  The  sea-caves  are  there  as  perfect  as 
those  on  the  adjacent  shore.  If,  again,  one  digs 
below  the  soil  of  the  terrace,  one  finds  a  layer  of 
typical  sea  sand  and  gravel  spread  over  the  solid 
rock,  out  of  which  one  may  pick  the  shells  of 
limpets,  periwinkles,  cockles,  and  oysters,  as  freely 
as  on  the  sands  of  the  existing  shore.  Clearly,  then, 
we  are  standing  on  an  old  beach,  an  old  terrace  cut 
out  by  the  sea.  But  when  it  was  cut  the  level  of  the 
sea  was  evidently  higher  than  now,  or  the  land 
lower.  Where  we  see  terrace  above  terrace,  we  are 
evidently  marking  the  steps  by  which  the  land  rose, 
or  the  sea  fell.  These  "  raised  beaches,"  therefore, 
testify  to  the  changes  of  relative  level  of  the  land 
and  sea. 

The  reader  may  be  disposed  to  ask  whether  it 
never  appears  that  the  sea  has  relatively  risen.  Un- 
fortunately the  answer  must  generally  be  sought 


MOUNTAIN  BUILDING  83 

below  the  sea.  As  we  find  on  the  dry  land  clear 
proofs  of  the  former  presence  of  the  ocean,  so  we 
must  search  below  the  sea  for  the  characteristic 
features  of  a  land-surface.  For  movements  of  small 
extent  in  this  direction  there  is  very  clear  evidence. 
In  the  estuary  of  the  Mersey,  for  example,  at 
Leasowe,  and  at  many  other  places  round  our 
coasts,  a  dead-low  tide  exposes  a  profusion  of  tree 
stumps  and  roots  on  the  shore.  The  stumps,  for 
the  most  part,  are  in  an  upright  position,  and  a  little 
digging  reveals  the  fact  that  they  are  naturally 
rooted  in  an  old  clayey  soil.  At  the  locality  named, 
also,  the  stumps  themselves  are  surrounded  by  peat ; 
this,  in  turn,  is  covered  by  another  soil,  at  about  the 
level  of  the  highest  tides,  in  which  Roman  remains 
may  be  found,  and  that  soil  is  itself  buried  under  the 
sand-dunes.  Here,  then,  the  proof  is  conclusive. 
The  forest  could  never  have  grown  had  the  older 
soil  always  been,  as  it  is  now,  permanently  covered 
by  the  sea.  The  land  has  there  sunk,  or  the  sea  has 
risen.  For  evidence  of  more  extensive  movements 
in  the  same  direction  we  may  appeal  again  to  the 
sedimentary  rocks  themselves.  Deposits  which  may 
have  been  formed  in  deep  water  obviously  cannot 
assist  us,  and  even  the  finding,  at  depths  much 
below  sea-level,  of  such  deposits  as  are  normally 
formed  in  shallow  water,  is  not  quite  conclusive ;  for 
very  coarse  materials  may  gather  in  deep  water 
where  the  land  dips  steeply  under  the  sea.  But, 
fortunately,  the  sedimentary  series  includes  oc- 
casional deposits  which  must  have  been  formed 
above  water  altogether.  The  most  familiar  and 
obvious  are  our  beds  of  coal.  Everyone  knows  that 
coal  is  merely  fossilised  wood  and  other  plant 
remains,  and  very  commonly  the  bed  of  clay  or 


84  GEOLOGY 

sandstone  below  the  layer  of  coal  in  the  pit  is 
crowded  with  roots  of  all  descriptions,  including 
frequently  the  roots  of  large  trees  in  their  natural 
position.  The  clay  is  evidently  the  old  soil  in  which 
the  plants  grew,  and  at  the  time  of  their  growth  it 
was  evidently  not  below  the  sea  (at  least,  not  more 
below  than  would  convert  it  into  a  swamp,  which 
very  probably  was  its  true  condition).  Yet,  in  parts 
of  each  of  our  great  coal  fields,  these  beds  are  now 
found  and  worked  at  depths  of  two  and  three 
thousand  feet  below  present  sea  level.  One  con- 
clusion is  now  amply  demonstrated :  the  phenomena 
are  in  no  wise  explained  by  the  supposition  that  the 
sea  has  gradually  subsided  and  the  land  emerged 
during  past  geological  ages.  While,  for  example,  in 
central  Derbyshire  we  have  limestones  which  ac- 
cumulated under  the  sea,  and  largely,  perhaps*  in 
deep  water,  now  rising  fourteen  hundred  feet  above 
sea-level,  we  have  also,  only  a  few  miles  away  in 
Lancashire,  the  beds  of  coal  now  three  and  four 
thousand  feet  below  it.  One  more  piece  of  cor- 
roborative evidence  may  be  added  to  amplify  the 
case.  The  land-surface  receives  its  characteristic 
outlines  at  the  hands  of  the  various  agents  of 
denudation,  and  it  is  naturally,  therefore,  far  more 
diversified  in  contour  than  the  bed  of  the  ocean. 
Hence  we  may  search  for  sunken  lands  by  their 
characteristic  contours  below  the  sea.  River  valleys 
are  the  most  characteristic  feature.  Nothing  re- 
sembling them  could  ever  be  produced  below  the 
sea;  yet  the  soundings  not  rarely  show  that  the 
actual  valleys  of  the  land  do  continue  below  the  sea, 
sometimes  for  considerable  distances  and  to  con- 
siderable depths.  So  far  as  the  true  valley  continues 
it  marks  the  former  extension  of  the  land,  but  the 


MOUNTAIN  BUILDING  85 

land  has  been  submerged  and  the  end  of  the  valley 
"  drowned." 

The  only  indication  among  the  facts  just  considered 
as  to  how  these  displacements  have  occurred  is  that 
given  by  the  series  of  raised  beaches.  Where  these 
are  seen  forming  step  above  step  it  is  evident  that 
after  each  alteration  of  level  there  was  a  pause  in 
the  movement  while  the  sea  cut  out  a  terrace.  The 
movement  in  these  cases  was  intermittent,  but  we 
are  still  left  in  doubt  whether  actual  displacements 
were  sudden  or  slow.  Some  further  light  may  be 
obtained  from  another  reference  to  the  coal-bearing 
strata.  In  Lancashire  those  rocks  are  altogether  up- 
wards of  5,000  feet  in  thickness,  and  include  several 
hundred  beds  of  coal,  most  of  which  are  much  too  thin 
to  be  of  any  value.  Most,  if  perhaps  not  all,  of  these 
seams  were  formed  at  or  about  sea-level — at  any 
rate  not  below  it.  The  layers  of  sandstone,  shale, 
and  clay  between  the  seams  are  such  as  to  indicate 
that  they  were  all  formed  in  shallow  water.  Now, 
seeing  that  the  uppermost  layers  of  rock  and  coal 
were  formed  at  or  near  sea-level,  the  lowest  must 
then  have  been  over  5,000  feet  below  it ;  but  they, 
too,  had  been  formed  at  sea  level.  The  land  must 
have  sunk,  or  the  water  risen,  5,000  feet  during  the 
interval ;  but  so  slow  and  so  gradual  must  have  been 
the  sinking  of  the  land  that  the  slow-growing 
sediments,  on  the  whole,  kept  pace  with  it.  There  is 
still  no  proof  that  the  sinking  might  not  have 
proceeded  by  jumps  of  a  few  feet,  or  even  fifty  feet, 
with  long  pauses  between  while  the  sediment 
"  caught  up,"  but,  averaging  the  whole  movement  of 
five  thousand  feet,  it  must  have  been  extremely  slow. 
An  inch  per  annum  would  almost  certainly  be  a  very 
excessive  estimate  for  the  rate  at  which  the  sediments 


\ 


86  GEOLOGY 

would  be  deposited,  and  the  average  rate  of  move- 
ment must  have  kept  below  their  rate  of  accumula- 
tion. The  story  which  we  have  just  worked  out  for 
the  Coal  Measures  is  only  that  which  is  repeated  by 
the  majority  of  the  sedimentary  rocks.  Again  and 
again  they  show  that  they  have  been  formed  during 
periods  of  slow  subsidence  of  the  land  relatively  to 
the  sea.  But  again  also  we  remember  that  the  very 
fact  that  the  land  of  to-day  consists  largely  of  such 
sediments  shows  that  there  have  been  periods  when 
the  movements  in  those  same  localities  have  been  in 
the  reverse  direction.  We  have  now  reached  two 
important  conclusions:  the  movements  were  not 
constant  in  direction,  and  those,  at  least,  whose 
duration  we  can  estimate,  occupied  enormous  periods 
in  their  performance. 

We  had  occasion  to  remark,  in  the  second  chap- 
ter, that  earthquakes  are  accompanied  occasionally 
by  permanent  changes  in  the  level  of  the  ground. 
These  changes  are  sudden,  but  they  are  never  large. 
In  minor  earthquakes  they  are  commonly  absent  or 
unmeasurable.  After  a  series  of  earthquakes  in 
Chili  during  1822  and  1835  the  coast  is  stated  to 
have  been  raised  from  two  to  four  feet  for  a  distance 
of  1,200  miles.  During  the  recent  San  Francisco 
earthquake  the  railway  along  the  coast  was  sub- 
merged for  three  miles.  A  series  of  shocks  affecting 
the  Indus  delta  in  1819  submerged  an  area  of  some 
2,000  square  miles,  while  an  adjacent  area  was 
elevated  10  feet.  We  have  already  seen  that  the 
vertical  displacement  along  the  line  of  fracture  in 
the  Mino-Owari  earthquake  in  1891  amounted  in 
places  to  as  much  as  20  feet.  This  last  figure  must 
be  regarded  as  very  exceptional;  even  greater 
displacement  occurred  during  the  disastrous  earth- 


MOUNTAIN  BUILDING  87 

quake  of  1692  in  Jamaica,  when  parts  of  Port  Royal 
(at  that  time  the  capital)  sank  20  to  40  feet,  but  it 
has  been  suggested,  with  much  probability,  that  this 
was  really  due  largely  to  the  "settling"  of  loose 
sand. 

Among  the  phenomena  of  earthquakes,  then,  we 
have  to  number  small  permanent  changes  in  the 
level  of  the  land.  Even  the  largest  are  minute 
compared  with  the  vast  movements  the  rocks  evince, 
but  if  they  be  indefinitely  repeated  in  the  same 
direction  they  may  obviously  mount  up  to  ultimate 
changes  of  any  magnitude.  We  have  already  ob- 
served that  the  periods  of  time  involved  must  be 
immense.  The  known  rate  at  which  denudation 
takes  place  gives  a  basis  for  the  estimation  of  that 
of  deposit,  from  which  we  may  fairly  conclude  that 
growth  in  thickness  at  the  rate  of  one  foot  in  500 
years  may  not  be  an  unfair  average,  while  one  foot 
per  century  may  certainly  be  regarded  as  rapid. 
Evidently  ordinary  earthquake  movements  will  not 
need  to  be  repeated  with  undue  frequency  to  keep 
pace  with  deposits  growing  even  at  the  latter  rate, 
while,  as  we  have  seen,  the  common  occurrence  of 
vast  thicknesses  of  sediment,  all  of  which  have  been 
deposited  in  shallow  water,  shows  that  the  rate  of 
deposit  has  usually  fully  kept  up  with  that  of  move- 
ment. Such  displacements  as  might  accompany 
very  small  earthquakes,  happening  a  few  times  in  a 
century,  would  suffice.  Indeed,  we  have  the  best 
reasons  for  believing  that  the  subsidences  and 
upheavals  may  in  many  cases  have  proceeded  by 
stages  so  gentle  that  no  shocks  at  all  marked  their 
occurrence.  A  tradition  has  for  a  very  long  period 
prevailed  among  the  inhabitants  of  the  Swedish 
coast  that  the  sea  is  gradually  receding  from  its 


88  GEOLOGY 

shores.  So  far  back  as  the  early  part  of  the 
eighteenth  century  the  celebrated  Celsius  brought 
forward  observations  supporting  the  contention,  and 
after  long  and  severe  criticism  it  has  been  firmly 
established  that  the  Gulf  of  Bothnia  is  actually 
subsiding  relatively  to  the  land,  though  not  every- 
where at  the  same  rate.  While  at  Stockholm  the 
fall  appears  to  be  less  than  six  inches  per  century, 
further  north  it  may  be  more  than  two  feet  in  the 
same  period.  On  the  other  hand,  in  the  south  of 
the  peninsula,  it  is  the  land  which  appears  to  be 
subsiding  relatively  to  the  sea.  The  truth  of  the 
matter,  of  course,  is  that  in  both  cases  it  is  the  land 
which  is  moving,  not  the  sea.  Were  the  level  of  the 
water  to  alter,  the  change  must  be  everywhere  alike. 
Here,  then,  we  have  elevation  and  subsidence  going 
on  so  gently  that  the  region  is  singularly  free  from 
earthquake  shocks,  and  yet  proceeding  quite  as 
rapidly  as  the  greatest  movements  appear  to  take 
place. 

If  we  turn  again  to  the  rocks  we  readily  perceive 
that  irregularity  of  movement  has  been  the  rule,  not 
the  exception.  The  evidence  is  of  the  simplest 
description.  Knowing  that  the  sedimentary  rocks 
have  been  laid  down,  sheet  upon  sheet,  under  water, 
we  can  have  no  hesitation  in  affirming  the  layers 
were  originally  horizontal.  Yet  we  rarely  find  them 
so  now.  Instead,  we  find  them  inclined  at  every 
angle ;  in  the  language  of  the  quarrymen  the  beds 
"  dip  "  in  some  direction.  Obviously,  if  the  rocks 
have  been  elevated  in  one  place  and  depressed  in 
another,  such  tilting  must  be  the  result.  Almost 
every  section  of  the  rocks  in  sea-cliff  or  quarry 
exhibits  such  tilting,  and  that  to  a  much  greater 
degree  than  might  have  been  anticipated.  A  gentle 


MOUNTAIN  BUILDING  89 

dip  might  be  expected,  but  what  are  we  to  say 
when  we  find  the  rocks  turned  absolutely  on  end. 
Nay,  further,  we  may  even  find  beds  turned  up 
and  forced  over  so  as  now  to  lie  upside  down. 
The  rocks  are  not  merely  tilted,  but  crumpled 
and  folded  like  a  rucked- up  carpet.  The  folds  may 


FIG.  6. — FOLD  IN  ROCKS  FORMING  CLIFF  WEST  OP  LITTLE 

HANGMAN  HILL,  NEAR  ILFRACOMBE.    The  rocks  on  the  left 

side  of  the  fold  have  been  completely  overturned. 

be  of  any  size.  Small  folds  can  nowhere  be  better 
seen  than  in  the  beautiful  cliffs  of  the  North  Devon 
coast.  Nothing  can  be  more  impressive  than  to 
walk  along  that  coast,  mile  after  mile,  seeing  the 
rocks  everywhere  twisted  into  folds  like  a  series  of 
great  waves,  folds  often  so  sharp  that  the  rocks  on 
one  side,  as  in  our  illustration,  are  actually  inverted. 


90  GEOLOGY 

The  mind  is  helpless  indeed  to  conceive  of  the  force 
which  can  thus  have  crumpled  the  solid  rocks  like 
sheets  of  paper ! 

The  greater  folds  of  the  rocks  are  naturally  not  to 
be  actually  seen  to  the  same  effect.  The  Pennine 
Chain  affords  a  very  perfect  example.  That  range 
of  hills  is  nothing  more  nor  less  than  the  worn-down 
remnant  of  a  great  arch  of  rock  thirty  miles  across, 
and  which,  if  the  material  worn  away  from  the  top 
could  be  replaced,  would  be  some  10,000  feet  higher 
in  the  middle  than  at  the  sides.  Here  we  have  a 
mountain  range  carved  out  of  a  single  great  fold  of 
the  earth's  crust.  But  the  great  ranges  of  the 
world  are  of  another  nature.  The  Grampian  Hills 
belong  to  the  class.  Wherever  we  turn  among  those 
mountains,  we  find  the  rocks  folded  and  contorted 
in  a  manner  which  would  be  perfectly  incredible 
were  it  not  there  before  our  very  eyes ;  folded  not 
only  in  vast  curves  miles  in  length,  but  all  minutely 
crumpled  even  down  to  microscopic  puckerings. 
The  accompanying  section  across  those  hills  only 
illustrates  the  greater  folds.  A  glance  at  it  will 
convince  the  reader  that  all  this  crushing  and 
crumpling  cannot  have  taken  place  without  causing 
a  general  elevation  of  the  whole  tract.  It  is  evident 
that  to  produce  all  the  folds  the  length  of  the  tract 
must  have  been  shortened  by  some  miles ;  and  what 
has  been  lost  in  length  must  have  been  made  up  for 
by  increase  in  height.  Of  this  nature  are  all  the 
greater  mountain-ranges  of  the  world.  They  are  the 
lines  of  weakness  of  the  earth's  crust,  along  which 
the  forces  of  deformation  have  been  concentrated 
to  crumple  up  the  rocks,  and  so  to  cause  a  great 
wrinkled  ridge  on  the  surface.  The  whole  line  has 
been  elevated  high  above  the  surrounding  tracts,  to 


gsouuna      I 


92  GEOLOGY 

become  a  prey  to  the  forces  of  denudation,  and  by 
them  to  be  carved  out  into  mountain  peak  and 
valley.  In  passing,  we  cannot  but  note  the  eloquent 
testimony  here  offered  to  the  power  of  denudation. 
By  following  the  foldings  of  the  rocks,  and  restoring 
the  parts  of  the  curves  which  have  been  worn  away, 
it  is  a  simple  matter  to  make  an  approximate 
estimate  of  the  amount  of  material  which  has  been 
removed.  Even  from  the  Pennine  Chain,  a  thick- 
ness of  six  or  eight  thousand  feet  of  rock  must  have 
been  denuded  from  the  top ;  from  the  Alps,  several 
miles  may  have  been  stripped  off. 

We  have  already  seen  that  it  is  not  only  in  the 
tilting  of  the  rocks  and  the  production  of  folds  that 
these  earth-movements  exhibit  their  effects.  In  the 
greater  earthquakes  we  generally  find  that  the  rocks 
have  snapped  along  some  line  and  been  displaced, 
producing  a  "fault."  The  number  of  such  faults 
whose  formation  has  thus  been  observed  is  rela- 
tively small ;  and  the  largest  displacement  we  have 
noted  has  been  only  about  twenty  feet.  Yet,  as  has 
already  been  remarked,  such  faults  are  among  the 
commonest  phenomena  of  the  rocks  all  the  world 
over.  It  may  be  doubted  if  there  is  a  square  mile 
in  the  whole  of  Britain  where  the  rocks  have  not 
been  displaced  in  this  way.  In  many  cases  the 
displacement  is  only  a  few  feet,  but  in  many  others 
it  is  to  be  measured  by  hundreds,  and  in  not  a  few 
by  thousands.  These  fractures  are  not  to  be  seen 
at  the  surface,  because  denudation  has  long  ago 
smoothed  away  the  irregularities  they  may  have 
created,  but  in  cliff  and  quarry  we  see  the  lines  of 
fracture  exposed,  and  we  can  estimate  the  displace- 
ment. The  greatest  fracture  in  Britain  cuts  across 
the  whole  of  Scotland  from  Stonehaven  to  the  Clyde, 


MOUNTAIN  BUILDING  93 

and  separates  the  hard  rocks  of  the  Grampians  from 
the  softer  beds  of  the  Central  Lowlands.  Along 
this  line,  the  displacement  may  well  amount  in 
places  to  10,000  feet.  What  shall  we  say  of  the 
force  which  has  caused  this  displacement  ? 

It  must  be  self-evident  to  all  that  the  formation  of  a 
great  fault  must  be  a  process  spread  over  the  same 
vast  intervals  of  time  that  are  occupied  in  the  slow 
movements  of  elevation  and  depression,  or  the  pro- 
duction of  great  folds.  Indeed,  it  is  clear  that  these 
phenomena  are  but  diverse  results  of  the  same 
movements.  When  one  area  is  elevated  or  de- 
pressed relatively  to  another  the  result  may  be  a 
general  tilting  of  the  rocks,  or  the  production  of  a 
more  or  less  sharp  bend  at  some  point,  or  the 
displacement  may  be  confined  entirely  to  a  single 
line  of  fracture.  Nor  is  this  true  only  of  vertical 
displacements.  In  speaking  of  the  effects  of  earth- 
quakes we  remarked  that  the  ground  is  frequently 
distorted  horizontally ;  and  we  correspondingly  find 
that  the  rocks  on  the  sides  of  a  fault  are  commonly 
displaced  laterally  as  well  as  vertically,  though,  in 
consequence  of  this  movement  being  more  difficult 
to  detect  it  is  frequently  quite  overlooked. 

The  most  stupendous  and  striking  of  all  rock- 
movements  have  been  in  a  horizontal  plane.  Along 
the  sides  of  the  great  mountain-chains,  where  the 
lateral  crushing  has  been  most  intense,  the  rocks 
have  not  uncommonly  been  torn  across,  and  the 
sides  of  the  range  bodily  crushed  in  under  the  more 
central  portions.  No  finer  example  is  known  than 
the  North-west  Highlands  of  Scotland.  The  whole 
of  what  is  now  the  coastal  area  has  been  thrust 
under  the  mountains  to  the  east.  Along  some  of 
these  great  "  thrust-planes,"  where  the  rocks  have 


94  GEOLOGY 

been  sheared  across,  the  overlying  mountain  masses 
have  been  forced  over  the  rocks  below  for  a  distance 
of  not  less  than  four  miles. 

We  have  now  sufficiently  illustrated  the  nature  of 
the  movements  by  which  the  lands  of  the  globe  are 
raise  \  again  from  the  ocean  grave  in  which  the 
forces  ol  denudation  strive  incessantly  to  bury 
them,  and  by  which  also  the  rocks  are  buckled  up 
along  their  lines  of  weakness  to  form  the  mountain 
ranges,  awaiting  only  the  action  of  sun  and  rain  to 
carve  them  into  peak  and  valley.  It  remains  for  us 
briefly  to  mark  the  effects  of  this  movement  on  the 
rocks  themselves,  and  to  consider  wherein  these 
stupendous  forces  may  reside. 

It  can  cause  no  one  surprise  to  learn  that  the 
rocks  are  profoundly  altered  by  the  enormous 
pressure,  crushing  and  shearing  to  which  they  are 
subjected.  The  most  familiar  example  of  these 
effects  is  in  the  conversion  of  clays  and  shales  into 
slates.  Not  merely  are  the  rocks  greatly  hardened, 
but  an  entirely  new  structure  is  developed  in  them, 
which  causes  them  to  split  or  "  cleave  "  readily  into 
thin  plates.  In  other  cases,  the  rocks  have  been 
completely  broken  up  into  larger  or  smaller  frag- 
ments, which  the  pressure  has  subsequently  com- 
pacted together  again.  Along  with  these  structural 
changes  there  is  usually  found  a  considerable 
chemical  reconstruction  of  the  rock.  The  old 
minerals  are  largely  destroyed,  and  new  ones 
are  built  up  out  of  their  materials.  This  latter 
change  cannot  be  due,  to  any  large  extent,  to  the 
pressure  itself,  but  is  probably  stimulated  by  it.  The 
ultimate  cause  of  the  change  is  to  be  found  in  the 
fact  that  the  constituents  of  the  rock  are  not 
(especially  in  the  case  of  the  sedimentary  rocks) 


MOUNTAIN  BUILDING  95 

in  chemical  equilibrium,  while    its    occurrence  is 

rendered  possible  by  the  heating  to  which  the  rock 
is  subjected  when  depressed  far  below  the  surface, 
and  by  the  presence  of  moisture,  through  which  the 
minerals  may  be  brought  particle  by  particle  into 
solution  and  subsequently  deposited  in  their  new 
combinations.  From  the  joint  action  of  all  these 
processes  there  results  a  whole  series  of  new  rocks, 
by  the  alteration  of  each  of  the  various  igneous  and 
sedimentary  types.  Granite  is  converted  into 
gneiss,  sandstone  into  quartzite,  limestone  into 
marble,  and  shales  and  clays  into  slates  and  schists. 
These  are  the  "  metamorphic  "  rocks. 

What  is  the  force  which  exhibits  itself  in  these 
vast  heavings  and  crumplings  of  the  earth's  solid 
crust?  The  geologist  to-day  answers  this  question 
much  more  cautiously  than  he  would  have  done  fifty 
years  ago.  The  comparison  of  the  earth  to  a 
shrivelled  apple  has  become  almost  a  tradition. 
The  outer  crust  of  the  globe  is  now  cold,  and  has 
been  for  many  millions  of  years.  The  interior  is 
hot,  but  is  slowly  losing  heat.  The  natural  inference, 
therefore,  is  that  the  interior  is  cooling,  and  if 
cooling,  shrinking.  The  cold  crust  cannot  contract, 
but  must  accomodate  itself  to  the  shrunken  interior 
by  wrinkling,  like  the  skin  of  the  apple.  Many 
difficulties,  however,  present  themselves  when  the 
theory  is  rigorously  tested.  The  amount  of  shrink- 
age of  the  interior  necessary  to  account  for  the 
observed  folding  of  the  crust  is  very  considerable, 
and  would  indicate  a  great  fall  in  temperature.  On 
the  other  hand,  the  amount  of  heat  now  escaping 
annually  can  be  fairly  estimated,  and  it  seems  very 
improbable  that  loss  at  such  a  rate  could  lead  to 
sufficient  cooling  in  any  period  of  time  it  would  be 


96  GEOLOGY 

reasonable  to  assume.  Another  element  of  un- 
certainty is  introduced  by  the  enquiry  whether  the 
lost  heat  may  not  be  restored.  We  know  now  that 
in  various  ways  it  may  be ;  whether  it  is,  we  cannot 
tell.  To  the  objection  regarding  the  inadequacy  of 
the  heat-loss,  it  may  be  very  legitimately  answered 
that  we  really  know  little  or  nothing  of  the  properties 
of  matter  under  such  conditions  of  temperature  and 
pressure  as  probably  exist  within  the  earth ;  but  that 
is  merely  a  dignified  way  of  saying  we  cannot  prove 
the  case.  The  second  objection  throws  doubt  on 
whether  any  case  exists.  For  the  present,  the  main 
support  for  the  theory  must  be  derived  from  the 
absence  of  any  completely  successful  alternative- 
While,  however,  the  suggestions  which  have  been 
made  are  too  complex  to  be  considered  here,  there 
remains  one  aspect  of  the  question  which  cannot  be 
entirely  passed  over.  Amid  the  endless  alternation 
of  subsidence  and  elevation  affecting  all  parts  of  the 
globe,  we  find  every  indication  that  the  great  con- 
tinents and  oceans  have  retained  their  identity.  In 
spite  of  every  effort  of  denudation  to  destroy  them, 
the  continents  are  still  pretty  much  where  they  were ; 
which  is  tantamount  to  saying  that  through  all  the 
vagary  of  rise  and  fall,  there  is  a  general  tendency 
for  the  continental  areas  to  rise,  while  the  ocean 
floors  sink.  The  very  fact  that  the  great  land-masses 
are  able  to  exist  at  all,  elevated  above  the  ocean 
floor,  seems  to  imply  that  they  must  consist  of 
relatively  light  materials ;  for  the  enormous  weight 
of  the  continental  protrusions  must  be  balanced  by 
something,  and  presumably  it  is  by  the  heavier 
character  of  the  rocks  below  the  ocean.  Now 
denudation  tends  to  destroy  this  balance;  by  its 
various  agencies,  enormous  masses  of  material  are 


THE  PHYSICAL  HISTORY  OF  BRITAIN         §fl 

transported  from  the  land  and  dumped  on  the  bed 
of  the  sea.  This  loss  of  balance  puts  a  strain  upon 
the  crust,  and  here,  perhaps,  we  may  see,  if  not  one 
of  the  chief  forces  which  cause  the  great  earth- 
movements,  at  least  one  of  the  guiding  principles 
which  direct  their  action. 


CHAPTER  VII 

THE   PHYSICAL    HISTORY   OF   BRITAIN 

WITHIN  the  narrow  limits  of  this  little  volume 
there  is  little  opportunity  to  enlarge  on  the  results 
achieved  by  geology  in  the  construction  of  a  detailed 
history  of  the  earth.  The  barest  indication  only 
can  be  given  of  what  has  been  learned  regarding 
the  past  fortunes  of  the  British  Isles  themselves ; 
but  this  is  perhaps  not  altogether  to  be  regretted. 
The  person  whose  interest  in  the  subject  ceases 
with  the  reading  of  geological  history  in  books  will 
be  but  poorly  repaid  for  his  pains.  In  proportion  as 
the  story  is  read  in  the  rocks  themselves  it  becomes 
vivid  and  absorbing. 

Let  us  consider,  then,  how  the  facts  we  have 
learned  are  to  be  applied  to  the  interpretation  of 
the  geological  record.  A  few  very  simple  consider- 
ations will  supply  the  key.  Since  the  sedimentary 
rocks  have  accumulated  layer  upon  layer  at  the 
bottom  of  the  sea,  it  is  evident  that  the  lowest 
layer  must  be  the  oldest,  and  the  uppermost  the 
youngest ;  and  herein  we  have  an  infallible  guide  to 
the  historical  order  of  the  nocks.  So  far  as  we  can 
see  how  the  beds  lie  one  above  another  we  can  read 
the  history  at  a  glance.  For  instance,  all  along  the 

G 


98  GEOLOGY 

eastern  flanks  of  the  Pennine  hills  we  may  see  the 
beds  of  blue  limestone  and  shales,  full  of  corals  and 
sea-shells,  "  dipping  "  to  the  east  and  passing  in  that 
direction  underneath  the  layers  of  coarse  yellow  grits 
and  black  shales  which  succeed  them,  and  in  which 
here  a  few  plant  remains,  there  a  few  shells,  may  be 
found.  The  limestones  take  us  back  to  a  time  when 
that  region  was  under  the  clear  waters  of  an  open 
sea,  in  which  corals  could  flourish.  The  grits  above 
tell  us  that  at  a  later  period  the  waters  became 
shallow,  and  great  rivers  swept  into  them  vast 
quantities  of  sand,  in  which  were  buried  not  only 
such  shells  as  could  live  there,  but  some  of  the 
fallen  trees  borne  out  from  the  land.  Above  the 
grits  in  turn  are  found  those  beds  of  sandstone  and 
clay  among  which  the  seams  of  coal  occur,  and  in  which 
shells  of  fresh-water  species  abound,  together  with 
the  remains  of  whole  forests  of  plants.  We  learn, 
then,  that  later  still  the  accumulating  sands  had 
driven  back  the  sea  altogether,  leaving  broad  swampy 
tracts  on  which  great  forests  sprang  up — a  swampy 
jungle.  From  time  to  time  the  land  subsided,  and 
fresh-water  lagoons  gathered,  till  fresh  sand  and 
mud  filled  them  up  and  allowed  a  new  forest  to  rise. 
In  the  case  just  considered,  the  story  is  clear  and 
straightforward.  But  where  the  rocks  have  been 
greatly  folded,  or  where  they  are  covered  for  long 
distances  by  superficial  beds  of  sand  or  clay,  and 
laid  bare  only  at  rare  intervals,  it  is  often  by  no 
means  so  easy  to  discover  what  is  the  real  order  of 
succession,  or  what  is  the  relation  of  the  particular 
bed  of  rock  seen  to  any  other  bed.  For  example, 
in  very  many  parts  of  Britain  isolated  patches  of 
limestone  may  be  found  which  are  scarcely  to  be 
distinguished  in  general  character  from  the  lime- 


THE  PHYSICAL  HISTORY  OF  BRITAIN         99 

stones  of  the  Pennine  Chain;  but  how  are  we 
to  know  whether  they  are  parts  of  those  lime- 
stones, or  whether  they  belong  to  any  of  the 
scores  of  other  limestones  which  represent  totally 
distinct  periods  of  the  world's  history?  The 
key  to  this  problem — one  of  the  most  important 
discoveries  in  the  history  of  geological  science — was 
found  by  an  engineer  of  Bath,  William  Smith, 
upwards  of  a  century  ago.  Having  traced  with 
great  care  the  distribution  of  all  the  various  beds  of 
rock  for  many  miles  around  that  town,  he  was 
enabled  to  make  the  all-important  discovery  that 
each  layer  of  rock  everywhere  contained  the  same 
assemblage  of  fossil  remains,  while  the  fossils  in 
any  group  of  rocks  were  always  different  from  those 
in  the  layers  above  or  below.  Thus  he  established 
the  proposition  that  strata  may  be  identified  by  their 
fossil  remains.  Applying  it  to  the  case  of  our 
limestones/we  see  that  it  is  only  necessary  to  examine 
the  fossils  from  the  various  isolated  patches  to 
decide  which  are,  and  which  are  not,  part  of  the 
same  series  as  those  in  the  Pennines.  We  see,  too, 
in  this  grand  discovery,  not  only  a  most  valuable 
guide  in  the  sorting-out  and  arrangement  of  the 
rocks,  without  which  the  history  of  the  world  could 
never  have  been  read,  but  also  a  record  of  the  fact 
that  the  inhabitants  of  the  earth  have  been  gradually 
and  constantly  changing,  each  period  of  its  history 
being  marked  by  its  own  peculiar  forms  of  life. 
Fossils,  then,  enable  us  to  piece  together  the  various 
isolated  sections  of  the  rocks  which  are  actually 
exposed  to  view,  and  to  build  up  the  whole  grand 
sequence  of  the  stratified  rocks  in  its  proper  order. 
We  find,  when  our  British  rocks  are  thus  placed  in 
order,  one  above  another,  they  form  a  pile  some 


100 


GEOLOGY 


twenty  miles  in  thickness.  If  the  average  rate  of 
accumulation  has  been  about  one  foot  in  five  hundred 
years,  the  reader  may  form  his  own  estimate  of  the 
ages  which  have  passed  while  these  deposits  grew. 
In  the  following  table,  the  greater  groups  of  the 
fossiliferous  rocks  are  arranged  in  historical  order, 
the  youngest  at  the  top. 

TABLE    OF    BRITISH    STRATIFIED    ROCKS. 


Cainozoic 


Mesozoic 


I  Pleistocene 
Pliocene 
Miocene 
Oligocene 
Eocene 


Cretaceous 

Oolitic 

Liassic 
Triassic 

Permian 


f  Chalk 

1  Greensand  and  Gault 

1  Wealden 

[_  Purbeckian 

Portlandian 

Kimmeridgian 

Corallian 

Oxford  ian 

Callovian 

Great  Oolites 

Inferior  Oolites 


f  Keuper 
1  Bunter 


[  Coal  Measures 
Carboniferous  j  Millstone  Grit 

(  Carboniferous  Limestone 
Old  Red 
Sandstone  and 
Devonian 

j  Ludlow        beds 

Palaeozoic     4  Silurian  j  Wenlock         „ 

I  Llandovery     „ 
f  Bala 

Ordovician        ]  Llandeilo        „ 
I  Arenig  „ 

{Tremadoc       „ 
Lingula 
Menevian       „ 
Harlech          „ 

Pre-Cambrian  unfossiliferous  rocks. 


Cambrian 


C  PHYSICAL  Hisfbfcv  OF'  BRITAIN          101 


102  GEOLOGY 

The  groups  of  rocks  named  in  the  foregoing 
table  are  extremely  unequal  in  thickness,  those 
nearer  the  base  of  the  series  being  in  general  much 
thicker  than  those  higher  up ;  and,  on  the  whole,  they 
doubtless  represent  correspondingly  greater  periods 
of  time.  But,  as  in  human  history  the  records  of  the 
later  centuries  are  much  more  perfect  than  those  of 
earlier  times,  so  it  is  with  the  records  of  the  rocks. 
The  further  down  the  series  we  go,  the  more  im- 
perfect as  a  rule  the  record  becomes.  Moreover,  we 
must  remember  that  the  geological  record  was  of 
necessity  incomplete  to  begin  with.  The  site  of  these 
islands  has  certainly  been  for  long  ages  below  the 
sea,  while  the  various  sedimentary  strata  accumula- 
ted ;  but  for  periods  equally  long,  in  all  probability, 
it  has  from  time  to  time  been  raised  above  the  waves. 
And  during  the  latter  times,  not  only  were  no  rocks 
formed,  but  those  which  had  already  gathered  were 
largely  destroyed  again  by  denudation,  till  subsidence 
once  more  carried  them  below  the  sea  and  the  old 
land  surface  was  buried  under  newer  sediments.  In 
the  interval,  the  older  rocks  have  not  merely  been 
denuded,  but  commonly  tilted  or  folded  as  well,  so 
that  the  newer  strata  have  frequently  been  laid  down 
horizontally  over  the  worn  edges  of  their  inclined 
beds,  as  in  the  accompanying  illustration.  Every 
such  "  unconformity,"  therefore,  tells  of  a  great  gap 
in  the  rocky  series;  of  an  interval  during  which  the 
lower  rocks  were  raised  above  the  sea,  folded,  and 
denuded.  The  older  rocks,  too,  have  naturally 
suffered  most  from  the  repeated  earth  movements. 
Folded  and  folded  again,  they  have  often  been 
crushed  almost  out  of  recognition,  and  their  fossils 
have  been  damaged  or  completely  obliterated. 
From  one  cause  or  another,  therefore,  the  geological 


THE  PHYSICAL  HISTORY  OF  BRITAIN        103 

record  is  most  fragmentary.  In  the  expressive 
language  of  Darwin,  "we  may  compare  it  to  a  history 
of  which  we  possess  only  here  and  there  a  chapter; 
of  each  chapter  only  a  few  pages ;  and  of  each  page 
but  a  few  lines." 

With  the  foregoing  considerations  in  mind,  then» 
let  us  turn  again  to  the  rocks  and  trace  some  of  the 
grander  features  of  this  country's  history.  The 
oldest  rocks  of  these  islands,  the  foundations,  so  to 
speak,  on  which  everything  else  rests,  are  to  be 
found  in  the  extreme  north-west,  forming  the  rugged 
coasts  of  North-western  Scotland,  and  the  whole 
of  the  Outer  Hebrides.  That  region  is  merely  a 
continuation  of  the  great  plateau  of  Scandinavia,  a 
remnant  of  the  ancient  axis,  the  old  "back-bone"  of 
Europe.  Along  that  line,  the  rocks  have  been  more 
intensely  crushed  and  folded  than,  perhaps,  anywhere 
else  in  the  Old  World.  And  so  our  history  begins, 
as  is  inevitable,  among  its  greatest  difficulties.  The 
rocks  are  so  altered  that  scarcely  a  trace  of  their 
original  character  is  to  be  found.  They  are  so 
folded  that  no  order  in  their  sequence  remains. 
It  is  nearly  certain  that  they  never  had  any  fossils, 
but  if  any  were  there  no  sign  of  them  is  left.  We 
believe  them  now  to  have  been  for  the  most  part 
igneous  rocks  of  a  volcanic  nature,  vast  sheets  of 
lava,  the  products  of  immense  primaeval  eruptions. 
The  one  fact  which  is  abundantly  evident  is  that 
these  "Lewisian"  rocks  (after  the  island  of  Lewis) 
were  very  early  subjected  to  the  most  powerful 
earth  movements,  and  folded  and  altered  to  almost 
as  great  a  degree  as  they  are  at  present,  long 
before  Cambrian  times.  After  their  folding,  further 
volcanic  activity  is  evinced  by  innumerable  "dykes" 
of  lava  which  penetrate  them.  In  turn,  we  find 


104 


GEOLOGY 


FIG.  10,— GEOLOGICAL  MAP  OF  THE  BRITISH  ISLES. 


THE  PHYSICAL  HISTORY  OF  BRITAIN        105 

evidence  of  a  second  period  of  folding,  which 
affected  the  dykes  as  well  as  the  older  rocks. 
Then  there  must  have  followed  an  immense  space 
of  time  while  all  these  rocks  were  exposed  to  the 
ceaseless  attack  of  the  elements,  by  which  the 
great  mountain  range  into  which  they  had  been 
folded  was  worn  down  to  its  roots.  There  is  some 
evidence  that  almost  desert  conditions  prevailed, 
and  the  valleys  were  slowly  filled  with  the  debris 
from  the  hills.  So  accumulated  the  vast  piles  of 
sand  and  gravel,  in  places  not  less  than  10,000 
feet  thick,  which  now  form  many  of  the  drear 
forbidding  hills  of  the  North-west  Highlands. 
Below  these  sheets  of  now  solid  rock  we  may 
still  follow  the  outlines  of  some  of  the  old  land 
they  buried,  the  oldest  surface  of  this  country  of 
which  any  trace  remains. 

What  lapse  of  ages  the  events  just  narrated  may 
represent  we  have  little  means  to  judge.  We  only 
know  that  they  had  all  transpired,  and  the  "Torri- 
donian"  sands  and  gravels  (from  Loch  Torridon) 
had  become  hard  rock,  and  been  themselves  carved 
out  into  hill  and  valley,  before  the  beginning  of 
the  Cambrian  period.  We  know,  too,  that  in  other 
parts  of  the  country  great  changes  had  occurred. 
Subsequently,  in  all  probability,  to  the  formation 
of  the  Lewisian  rocks,  the  whole  area  of  these 
islands,  perhaps,  had  been  below  the  sea,  and  in 
that  sea  were  deposited  the  sheets  of  sand  and 
mud,  with  bands  of  limestone  which  now  form 
almost  the  whole  of  the  Grampian  highlands,  as 
well  as  the  ancient  rocks  which  in  many  other 
parts  of  the  country  peep  out  from  below  the 
younger  sediments.  But  these  rocks,  too,  had 
suffered  so  profoundly  from  the  earth-movements 


106  GEOLOGY 

that  their  primitive  character  is  almost  entirely 
lost.  The  shales  were  altered  to  slates  and  glitter- 
ing crystalline  schists,  the  limestones  to  marbles, 
the  sandstones  to  hard  quartzite.  We  cannot  but 
believe  that  they  once  enclosed  the  fossil  remains 
of  the  strange  creatures  of  these  early  oceans. 
Yet  no  trace  of  them  is  now  to  be  found. 
Volcanoes  poured  out  their  lavas  over  the  sea-beds, 
or  intruded  them  among  the  sediments,  while  deeper 
down,  the  intruded  molten  masses  cooled  to  form 
the  granites  of  to-day.  Among  Britain's  truly 
ancient  mountains  we  have  the  mutilated  and 
almost  obliterated  records  of  the  early  history  of 
the  country.  Here  and  there  we  may  decipher  a 
fragment,  but  most  of  the  story  of  those  early 
times  must  remain  forever  a  part  of  the  great  un- 
known. 

From  the  lowest  Cambrian  rocks  upwards,  we 
have  the  invaluable  aid  of  the  fossils  to  guide  our 
enquiries,  and  the  main  features  of  the  subsequent 
history  are  known  with  certainty.  Before  that 
period  began,  it  is  clear  that  all  the  older  rocks 
had  been  greatly  worn  down  by  the  elements. 
Wherever  the  base  of  the  Cambrian  system  can  be 
traced,  it  rests  on  an  irregular  surface  of  the  rocks 
below — the  second  surface  which  we  know  this 
country  to  have  possessed.  The  Torridonian  rocks, 
for  example,  had  been  in  many  places  completely 
worn  away  again.  The  Cambrian  rocks  themselves 
consist,  in  Wales,  from  whose  classical  name  their 
title  is  derived,  of  sandstones  and  slates  (the  latter 
altered  shales)  containing  a  great  variety  of  marine 
fossils.  In  North-west  Scotland,  nearly  two  thou- 
sand feet  of  limestone  is  included  in  the  system. 
On  the  whole,  however,  we  may  be  confident  that 


THE  PHYSICAL  HISTORY  OF  BRITAIN         107 

these  deposits  were  accumulated  in  fairly  shallow 
water,  and  as  they  reach  in  Merioneth  a  thickness 
of  upwards  of  twelve  thousand  feet,  they  therefore 
clearly  point  to  a  gradual  subsidence  of  the  sea- 
bed. And  further,  as  these  deposits  occur  in  the 
south  and  north  of  Wales,  in  Shropshire,  and  the 
Malvern  Hills,  in  parts  of  Ireland  and  north  of 
Scotland,  it  is  fairly  clear  that  the  Cambrian  sea 
must  have  extended  over  the  greater  part  of  these 
islands,  though  the  rocks  are  now  in  most  parts 
either  hidden  under  newer  deposits,  or  completely 
worn  away  again. 

The  Cambrian  subsidence  continued  without 
serious  interruption  throughout  Ordovician  and 
Silurian  times,  and  to  such  an  extent  that  no  less 
than  sixteen  thousand  feet  of  the  former  and  five 
thousand  feet  of  the  latter  rocks  were  deposited. 
The  fineness  of  the  mud  which  forms  some  of  the 
slates  and  shales  of  these  systems  would  seem 
even  to  indicate  that  the  water  over  some  areas 
was  at  times  of  considerable  depth.  Yet  the  evidence 
that  some  areas  remained  above  the  water,  in 
spite  of  the  prolonged  denudation  to  which  they 
must  have  been  subjected,  proves  also  that  the 
subsidence  was  not  universal.  In  places,  the  country 
must  have  been  actually  rising,  and  we  must  con- 
clude, as  usual,  that  what  was  really  occurring  was 
not  a  bodily  subsidence  of  the  whole  region,  but 
rather  a  slow  folding  of  the  earth's  crust.  There 
is  no  doubt,  however,  that  during  this  immensely 
long  period,  the  greater  part  of  the  British  area 
remained  almost  continually  below  the  sea,  with 
only  scattered  islands  peering  above  its  waters; 
by  far  the  longest  period  of  uninterrupted  submer- 
gence of  which  we  have  a  certain  record.  The 


108  GEOLOGY 

Ordovician  rocks,  which  attain  their  greatest  develop- 
ment in  Central  Wales  (and  are  named,  therefore, 
after  the  territory  of  the  ancient  Ordovices)  consist, 
like  the  Cambrians,  mainly  of  sandstones  and  slates, 
the  famous  slate-quarries  of  North  Wales  begin 
mainly  opened  in  them.  Among  the  Silurian  beds 
(from  the  country  of  the  Silures,  to  the  east  of 
Wales)  several  thick  bands  of  limestone  occur, 
often  full  of  fossil  corals.  So  abundant,  indeed, 
are  the  corals,  that  some  of  the  beds  have  been 
well  described  as  fossil  coral-reefs. 

The  advent  of  the  Ordovician  period  was 
marked  by  great  and  widespread  outbursts  of  vol- 
canic activity.  All  over  North  Wales  and  in  the  Lake 
District  volcanoes  arose.  Nearly  the  whole  of  the 
Cader  Idris  range,  and  much  of  the  Arenig  and 
Snowdon  ranges,  is  composed  of  sheets  of  lava 
and  volcanic  ash  ejected  at  this  period.  In  places, 
the  aggregate  thickness  of  the  volcanic  materials 
is  not  less  than  six  thousand  feet.  The  volcanoes 
clearly,  were  of  the  first  magnitude.  Yet  the 
eruptions  of  the  Lake  District  far  exceeded  those 
of  Wales.  By  far  the  greater  part  of  the  Cumbrian 
group  of  hills  is  carved  out  of  a  continuous  mass 
of  volcanic  material  which  is  estimated  at  from 
ten  to  fourteen  thousand  feet  in  thickness.  Even 
the  portion  of  the  lavas  now  exposed  extends  over 
an  area  about  twenty  miles  by  twenty-five,  and  this 
is  evidently  but  a  small  part  of  their  original  extent. 
Thus,  as  one  stands  on  the  summit  of  Scawfell  or 
Helvellyn  (both  carved  entirely  out  of  this  material) 
and  looks  over  the  surrounding  hills,  one  sees  on 
every  hand  the  products  of  one  of  the  greatest 
volcanic  outbursts  this  country  has  ever  witnessed. 
Throughout  the  Ordovician  period  local  eruptions 


THE  PHYSICAL  HISTORY  OF  BRITAIN        109 

continued,  but  the  first  great  effort  was  never  again 
equalled.  By  the  Silurian  epoch,  quiet  was  com- 
pletely restored. 

The  prolonged  subsidence  of  most  parts  of  this 
country,  which  had  kept  them  for  ages  below  the 
open  sea,  at  length  ceased,  or  at  least  became  slower 
and  less  general.  The  waters  became  everywhere 
shallow  and  perhaps  land-locked,  and  the  creatures 
of  the  open  sea  were  no  longer  able  to  live  in  them. 
Under  these  conditions  thick  masses  of  coarse  sand- 
stone and  marl  accumulated,  in  those  areas  where 
subsidence  continued,  to  form  the  Old  Red  Sand- 
stone system.  The  formation  of  these  rocks  may 
have  been  comparatively  rapid,  seeing  that  the  area 
of  land  exposed  to  denudation  in  the  immediate 
neighbourhood  was  now  much  greater  than  during 
the  preceding  periods.  Yet,  when  we  find  these 
sandstones  sometimes  upwards  of  16,000  feet  in 
thickness,  we  must  allow  that  they  represent  an 
immense  lapse  of  time.  During  this  period,  again, 
the  volcanic  fires  broke  loose,  this  time  principally 
in  Central  Scotland ;  and  the  Sidlaw  and  Ochil  Hills, 
among  others,  are  mainly  composed  of  their  lavas 
and  ashes.  One  part  of  the  country  remained 
throughout  below  the  open  sea — the  extreme  south. 
In  that  region  beds  of  sand,  mud  and  limestone, 
full  of  marine  shells  and  corals,  continued  to  gather, 
while  elsewhere  the  Old  Red  Sandstone  was  form- 
ing. Everyone  must  be  familiar  with  the  beautiful 
so-called  "  marbles "  of  Devonshire,  so  largely 
quarried  and  used  in  polished  slabs  for  ornamental 
purposes,  which  are  the  limestones  of  this 
"  Devonian  "  system. 

The  continued  earth-movements  at  length  raised 
the  entire  country,  except  possibly  the  south,  com- 


110  GEOLOGY 

pletely  above  sea-level.     For  the  third  time,  at  least, 
the  British  area  became  part  of  the  great  continent, 
The  Old  Red  and  Devonian  rocks,  along  with  all  the 
older  ones,  were  greatly  folded,  and  exposed  for  a 
prolonged  period  to  denudation.      In  many  parts, 
thousands  of  feet  of  rock  were  worn  away  during 
this  "  continental "  period,  and  the  whole  region  was 
carved  anew  into  hill  and  valley.     Hence  it  was  a 
very  uneven  land-surface  which  at  length  subsided 
again  below  the  ocean,  and  over  which  in  due  course 
the  Carboniferous  rocks  were  laid  down.    The  sub- 
mergence began  in  the  south,  and  there  the  corals, 
sea-lilies  and  shells  began  to  build  up  the  Carbon- 
iferous limestone  while   the  central  and   northern 
parts  of  Britain  were  still  above  water.    The  shore- 
line   crept    slowly  northward,  the   advancing  sea 
converting    the   hills  into  islands,   and   ultimately 
submerging  them.     At  the  foot  of  the  -Grampians 
the  onward   march  of  the   sea  was   stopped,   and 
throughout  the  Carboniferous  period  great  masses 
of  land  persisted  in  that  direction.     The  succession 
of  events  during  this  age  has  already  been  noted — 
the  shallowing  of  the  sea,  and  consequent  covering 
of  the  limestone  with  great  sandbanks,  now  forming 
the  Millstone  Grit,  followed  by  the  conversion  of  the 
whole  tract  into  a  swampy  jungle  which  periodically 
subsided,  but  was  repeatedly  brought  above  water 
again  by  new  accumulations  of  sand  and  mud,  and 
whose  successive  coverings  of  vegetation  are  now 
preserved  to  us  as  beds  of  coal.     Already,  before 
this  formation  of  the  Coal  Measures  was  complete, 
renewed  "  warping  "  of  the  country  began,  raising 
some    areas    into    high    land,  while    others    were 
depressed  and   became   covered  with    land-locked 
arms  of  the  sea.    The  Pennine  Chain  is  one  of  the 


THE  PHYSICAL  HISTORY  OF  BRITAIN        111 

ridges  produced  at  this  period.  In  the  enclosed  seas 
the  Permian  rocks  gathered.  To  the  east  of  the 
Pennine  ridge  some  limestone  was  formed,  but  to 
the  west  bright  red  sandstones  and  clays  alone 
gathered.  Meanwhile,  the  ridged-up  Carboniferous 
rocks  were  extensively  denuded,  and  the  Permian 
strata  in  many  parts  rest  over  their  worn  edges. 
The  waters  of  the  enclosed  seas  became  intensely 
salt,  supporting  only  a  dwarfed  assemblage  of  living 
creatures  and  leaving  deposits  of  salt  and  gypsum 
here  and  there  among  the  sediments. 

Again  Britain  was  raised  completely  beyond  the 
reach  of  the  waves,  in  common  with  all  Central  and 
Northern  Europe,  for  its  fourth  known  period  of 
continental  existence.  So  far  was  the  region 
thoroughly  continental  at  this  time,  that,  for  a 
lengthy  period,  desert  conditions  appear  to  have 
prevailed.  In  this  desert,  the  "Bunter"  ("mottled") 
sandstones  of  the  Triassic  system  were  formed. 
This  soft  sandrock,  with  its  warm  yellow  and  red 
tints,  is  a  well-known  feature  in  the  Midland  counties 
and  elsewhere.  Subsequently  a  great  Y-shaped  lake 
gathered  in  the  desert,  its  arms  enclosing  the  Pen- 
nine area,  and  its  "  tail  "  reaching  at  least  as  far  as 
our  present  south  coast.  Perhaps,  like  the  present 
Lake  Chad  in  the  Sudan,  it  was  a  mere  film  of  water 
which  often  entirely  vanished.  That  it  did  so  at 
times  is  sufficiently  proved  by  the  fact  that  the 
animals  of  the  period  roamed  about  and  left  their 
footmarks  all  over  its  bed.  Certainly  it  became 
intensely  salt,  and  the  famous  salt-beds  of  Cheshire 
were  deposited  from  its  waters.  Its  chief  relic  is 
the  thick  mass  of  bright  red  clay  which  forms  the 
upper  portion  of  the  Triassic  system. 

Throughout  the  remainder  of  the  Mesozoic  epoch, 


112  GEOLOGY 

Britain  was  reduced  to  an  essentially  insular  condi- 
tion, though  no  doubt  it  was  now  and  then  connected 
with  such  parts  of  the  continent  as  happened  to  be 
above  water.  Subsidence  during  this  period  took 
place  mainly  in  the  eastern  and  southern  parts  of 
England,  while  the  rest  of  the  British  area  was 
doubtless,  on  the  whole,  rising ;  only  small  parts  of 
it  being,  from  time  to  time,  submerged.  In  the 
south-eastern  sea,  beds  of  sand,  clay  and  limestone 
gathered  in  the  most  varied  succession,  pointing  to 
an  oscillating  sea-level,  and  giving  rise  to  the  varied 
series  of  rocks  which  now  characterise  that  portion 
of  the  country. 

During  the  Liassic  part  of  this  period,  the  sea 
spread  widely  over  these  islands,  but  later,  in  early 
Oolitic  times,  the  land-area  became  more  extensive. 
The  north-east  of  England  at  this  time  was  the  site 
of  a  great  estuary,  till  subsidence  once  more  gained 
the  upper  hand  in  that  area,  and  carried  it  below  the 
sea  of  the  Upper  Oolites.  At  the  beginning  of  the 
Cretaceous  period  the  conditions  were  reversed. 
The  south-east  of  the  country  was  now  an  estuary, 
opening  eastwards,  while  the  more  northern  parts 
were  under  deep  water.  This  latter  state  of  affairs 
was  shortly  followed  by  the  closing  scene  of  the 
Mesozoic  epoch,  when  all  the  centre  and  south  of 
Europe,  levelled  down  by  the  long-continued 
denudation,  sank  deeply  to  be  covered  by  a  great 
"  epi-continental  "  ocean — the  ocean  of  which  our 
present  Mediterranean  is  the  last  diminished 
remnant.  On  the  bed  of  this  ocean  there  gathered 
vast  deposits  of  fine  grey  ooze,  in  essential  char- 
acters like  the  ooze  which  is  gathering  to-day  on  the 
bed  of  the  Atlantic,  and  formed,  like  it,  mainfy  of 
countless  myriads  of  the  microscopic  shells  of 


THE  PHYSICAL  HISTORY  OF  BRITAIN        113 

Foraminifera.     That    ooze  now  forms    the  white 
chalk  cliffs  of  Albion. 

We  come  now  to  the  thoroughly  modern  ages  of 
geological  history.  The  general  subsidence  of  the 
Chalk  period  was  closely  followed  by  an  equally 
wide-spread  elevation.  Europe  resumed  its  full 
continental  form,  and  included  Great  Britain  within 
its  bounds.  Very  extensive  geographical  changes 
clearly  took  place,  which  resulted  in  the  invasion  of 
Europe  by  entirely  new  forms  of  life,  both  animals 
and  plants.  For  a  very  long  period  Britain  remained 
entirely  continental.  The  Eocene  rocks,  however, 
mark  the  encroachment  of  the  sea  over  the  south-east 
of  the  country.  At  first,  continuous  shallow  sea 
stretched  from  the  H umber  to  Portland,  but  later 
it  was  divided  by  the  elevation  of  a  ridge  which  now 
forms  the  watershed  between  the  Thames  basin  and 
the  rivers  of  the  south.  The  rising  of  this  arch 
above  the  waves  was  the  first  sign  of  a  series  of 
great  earth-movements  which  culminated  in  Miocene 
times.  In  Britain  itself,  the  chief  result  of  these 
movements  was  the  formation  of  this  fold  (of  which 
the  North  and  South  Downs  are,  so  to  speak,  the 
worn  edges)  and  the  elevation  of  the  entire  country 
above  the  sea  before  the  Miocene  period.  But  on 
the  continent,  far  more  stupendous  effects  were 
produced :  nothing  less,  in  fact,  than  the  formation 
of  the  Alps  themselves,  which  were  then  raised  to 
their  present  eminence,  or  perhaps  much  higher. 
Could  we  have  been  present,  nevertheless,  it  would 
not  have  been  the  raising  of  the  Alps  which  would 
have  impressed  us.  Apart  from  earthquakes,  that 
would  have  been  imperceptible.  On  our  own 
western  shores  much  more  striking  phenomena 
would  have  borne  witness  to  the  unquiet  state  of 

H 


114  GEOLOGY 

the  earth.  From  Ireland  to  Iceland  one  long  line 
of  volcanoes  broke  out,  with  all  the  vigour  which 
characterises  a  series  of  eruptions  following  a  long 
period  of  inactivity — for  the  whole  Mesozoic  period 
had  been  unmarked  by  a  single  outbreak.  Indeed, 
unable  to  escape  rapidly  enough  through  single 
vents,  the  lava  rose  at  times  simultaneously  through 
countless  fissures  in  the  rocks,  and  buried  thousands 
of  square  miles  of  country  under  vast  lava-floods. 
Almost  the  whole  of  the  county  of  Antrim  is  under- 
lain by  one  of  these  lava-sheets,  or  rather  by  a  small 
remnant  of  one,  while  Arran,  Mull,  Skye,  and  many 
other  of  the  Western  Isles  of  Scotland  are  largely 
the  worn-down  stumps  of  the  volcanoes. 

We  have  noted  that  during  Miocene  times  Britain 
was  entirely  above  the  waters  of  the  sea.  When 
East  Anglia  sank  again  under  the  early  Pliocene 
sea,  considerable  changes  had  occurred  in  the 
marine  life  since  Oligocene  times.  While  at  the 
latter  period  the  local  sea-fauna  had  an  almost 
tropical  aspect,  it  had  now  become  temperate  in 
character.  The  climatic  change  continued  through- 
out the  Pliocene  period.  At  its  close,  the  climate 
was  almost  arctic,  and  became  thoroughly  so  in  the 
early  stages  of  Pleistocene  history.  We  may  imagine 
the  snow  gathering  on  our  mountain-tops,  and  the 
glaciers  forming  and  creeping  ever  further  down 
their  sides.  The  valleys  were  filled  with  ice,  which 
united  in  vast  sheets  over  the  lowlands  till  the  whole 
country  north  of  the  Thames  and  Severn  was  buried 
under  one  vast  ice-field  as  completely  as  modern 
Greenland.  All  Central  and  Northern  Europe  lay 
under  the  same  icy  mantle,  and  the  British  ice 
united  with  that  of  Scandinavia  over  the  bed  of  the 
North  Sea.  For  many  thousands  of  years  the 


THE  PHYSICAL  HISTORY  OF  BRITAIN        115 

surface  of  this  country  was  ground  and  polished 
under  the  slowly  streaming  field  of  ice,  and  the 
smooth  outlines  which  now  characterise  British 
scenery  are  certainly  due  in  no  small  measure  to  this 
Great  Ice  Age.  While  the  ice  still  persisted,  a 
general  submergence  of  the  lowlands  appears  to 
have  occurred.  Into  the  water  the  ice  dropped  pell- 
mell  its  burden  of  sand,  mud  and  boulders,  thus 
covering  the  country  with  that  superficial  coating  of 
sand  and  gravel  which  to-day  hides  the  solid  rocks 
almost  everywhere  except  among  the  hills,  and  out 
of  which  thousands  of  ice-scratched  boulders  may 
be  gathered.  Slowly  the  country  rose  again,  the 
climate  ameliorated,  and  the  ice  shrank  back  into 
the  hills.  During  their  retreat,  the  glaciers  left 
many  a  moraine  of  debris  behind  them,  still  to  be 
recognised  in  hundreds  among  our  highland  valleys. 
The  sea  left  its  mark  in  the  raised  beaches  round 
our  coasts,  and  so  the  country  reached  the  condition 
in  which  we  now  find  it.  The  story  of  Geology  is 
ended ;  the  work  of  the  antiquarian  and  historian 
begins. 


116  GEOLOGY 


CHAPTER    VIII 

THE    HISTORY   OF    LIFE   ON   THE    EARTH 

BRIEF  and  fragmentary  as  these  sketches  of 
Earth-history  necessarily  are,  we  cannot  conclude 
them  without  a  reference  to  the  most  fascinating 
story  which  the  rocks  contain — that  of  the  life  of 
former  ages.  We  saw  in  the  preceding  chapter  that 
each  great  stratum  of  the  sedimentary  rocks  con- 
tains its  own  peculiar  assemblage  of  fossil  remains, 
which  differ  from  those  of  any  other  stratum,  but 
are  generally  most  similar  to  those  in  the  rocks 
immediately  above  or  below.  Thus,  while  the 
organisms  found  fossilised  in  the  uppermost  (and 
therefore  newest)  strata  differ  very  slightly  from  the 
creatures  still  living  in  the  same  region,  those  in  the 
successively  lower  (and  therefore  older)  rocks 
appear  more  and  more  unlike  the  existing  forms, 
until,  in  the  oldest  rocks,  we  find  ourselves  among 
the  most  strangely  unfamiliar  and  most  primitive 
creatures.  Moreover,  in  our  receding  glance,  we 
lose  sight  of  whole  groups  of  animals  one  after 
another ;  and  it  is  always  the  most  highly  developed 
creatures  which  are  the  first  to  be  lost  in  our 
backward  march.  These  few  simple  facts — that  the 
inhabitants  of  the  world  have  gradually  changed, 
that  there  has  been  a  constant  approximation  to  the 
forms  now  living,  and  that  the  groups  of  animals 
low  down  in  the  scale  of  life  have  always  appeared 
before  those  of  a  higher  organisation — are  the  grand 
features  of  the  support  given  by  Palaeontology  (as 


TYPICAL  FOSSILS. 


1.  Trilobite  crustacean.     Calymene  Blumenbachii, 

Wenlock  Limestone. 

2.  Brachiopod.     Spirifer  striatus,  Carboniferous  Limestone. 

3.  Cephalopod.     Ammonites  (Aigoceras)  capricornus,  Lias. 

4.  Echinoid.     Cidaris  florigemma,  Corallian. 

5.  Gastropod.     Valuta  luctatrix,  Eocene. 

6.  Pelecypod.     Cardita  planicosta,  Eocene. 

Plate  IV. \  [Geotogv,  116. 


THE  HISTORY  OF  LIFE  ON  THE  EARTH      117 

the  study  of  extinct  creatures  is  called)  to  the 
doctrine  of  Evolution.  If  life  has  been  continuous 
on  the  earth  from  the  beginning,  and  if  every  new 
form  which  has  appeared  has  resulted  from  the  slow 
modification  of  some  preceding  form,  then  the  signifi- 
cance of  those  facts  is  clear ;  if  not,  then  the  whole 
of  palaeontology  and  biology  is  unintelligible  and 
incapable  of  scientific  explanation.  With  these  few 
introductory  remarks,  let  us  turn  to  the  rocks  to 
learn  a  few  of  the  facts  they  have  for  so  many  ages 
preserved  for  us. 

In  the  most  ancient  strata  in  which  fossils  are 
still  to  be  found  in  a  recognisible  condition,  we  meet 
already  with  a  large  and  varied  assemblage  of 
organisms.  Nearly  all,  that  is  to  say,  of  the  great 
groups  of  animals  lower  in  the  scale  of  life  than  the 
Fishes,  are  represented,  though  for  the  most  part 
by  forms  very  different  to  those  now  living.  The 
question  naturally  arises,  therefore,  how  all  these 
forms  came  to  be  evolved  so  early  ?  In  reply,  the 
geologist  points  to  those  great  masses  of  altered 
rocks,  for  the  most  part  crushed  and  twisted  almost 
out  of  recognition,  of  which  we  can  only  say  that 
they  were  formed  before  the  Cambrian  period. 
That  many  of  them  once  contained  relics  of  still 
earlier  life,  now  destroyed,  is  practically  certain,  and 
the  high  development  of  life  in  Cambrian  times  only 
confirms  what  on  other  grounds  seems  probable — 
that  many  as  must  be  the  millions  of  years  which 
have  elapsed  since  the  Cambrian  rocks  were  formed, 
the  period  which  preceded  it  must  have  been  fully 
as  long.  All,  or  nearly  all,  of  the  history  of  those 
earlier  times  has  been  irretrievably  lost  by  the 
alteration  of  the  rocks  and  consequent  obliteration 
of  their  records. 


118  GEOLOGY 

The  lowliest  organisms  have  played  no  inconsider- 
able part  in  the  past  history  of  the  world.  In  the 
oldest  fossil-bearing  rocks  the  almost  microscopic 
shells  of  the  Foraminifera  are  found,  while  they 
have  contributed  not  a  little  towards  the  building  of 
the  limestones  of  various  ages.  The  Chalk,  we  have 
already  seen,  consists  not  uncommonly  mainly  of 
their  remains,  though  some  thousands  of  their  shells 
may  be  gathered  in  a  thimble;  and  the  Tertiary 
limestones  of  Egypt  (of  which  the  pyramids  are 
built),  and  some  of  the  Eocene  limestones  in  the 
neighbourhood  of  Paris  and  elsewhere  in  Europe 
are  formed  largely  of  the  shells  of  giant  members  of 
the  race.  To  the  palaeontologist,  however,  the  fossil 
foraminifera  are  of  interest  mainly  as  illustrating 
the  persistence  of  lowly  forms  of  life.  While  the 
higher  creatures  come  and  go,  changing  more 
rapidly  in  proportion  to  their  complexity,  these 
simple  creatures  persist  from  age  to  age,  almost 
unchanged  by  the  hand  of  Time.  Even  in  the 
Palaeozoic  rocks,  the  foraminifera  have  a  remarkably 
modern  aspect,  some  of  the  forms  being  scarcely 
distinguishable  from  those  now  living,  while  the 
higher  creatures  whose  remains  are  preserved  in 
the  same  rocks  are  all  unfamiliar. 

The  Corals,  too,  we  have  noted  as  rock-building 
creatures  in  all  ages.  More  highly  organised  than 
the  Foraminifera,  they  have  undergone  greater 
changes  during  their  history.  The  group  which 
includes  the  typical  reef-builders  of  the  present  day 
did  not  appear  in  the  world  till  the  early  part  of  the 
Mesozoic  epoch.  Their  ancestors  were  already 
living  in  Ordovician  times,  and  certainly  long  before, 
but  they  were  in  some  respects  different  from  the 
modern  forms.  While  the  latter  build  skeletons 


THE  HISTORY  OF  LIFE  ON  THE  EARTH      119 

which  are  usually  radially  symmetrical,  their  palaeo- 
zoic ancestors  formed  skeletons  of  a  bilateral 
character,  i.e.,  with  distinct  right  and  left  halves. 
In  this  change  the  corals  exemplify  a  modification 
which  has  nearly  always  come  over  animals  which 
have  adopted  a  sedentary  mode  of  life.  Fossil 
corals  are  among  the  most  familiar  of  objects.  Our 
mantel-pieces  and  other  objects  of  coloured  "marble" 
(really  limestone)  seldom  fail  to  show  the  beautiful 
lace-like  forms  of  the  corals  they  contain.  Especially 
is  this  the  case  with  the  well-known  red  marbles  of 
Devonshire. 

The  "  precious  corals  "  of  to-day  belong  to  a  group 
which  has  long  since  passed  its  prime.  Though  now 
comparatively  rare,  their  ancestors  abounded  in  the 
Palaeozoic  seas,  and  contributed  largely  to  the  coral 
reefs  of  those  early  ages. 

Of  shell-fish  (apart  from  lobsters,  shrimps,  etc.) 
there  are  two  widely  distinct  classes.  The  Mollusca 
include  all  the  common  sea-shells  and  land-shells  of 
the  present  day,  and  are  now  in  the  prime  of  their 
existence.  The  Brachiopoda,  or  "lamp-shells,"  on 
the  other  hand,  are  rarely  seen  by  the  ordinary 
person  outside  a  museum.  Yet  it  is  to  the  latter 
group  that  by  far  the  greater  number  of  the  countless 
shells  found  in  the  Palaeozoic  rocks  belong.  (See 
Plate  iv,  fig.  2.)  We  have  here  another  group  of 
animals  whose  day  is  long  past ;  and  we  may  learn 
from  them  an  oft-repeated  story.  During  their 
prosperous  days  they  gave  rise  to  many  varied  and 
complicated  forms;  yet  the  few  which  survive  are 
relatively  simple.  Again  and  again  we  are  faced 
with  the  same  fact.  Highly  complex  and  specialised 
organisms  may  succeed  in  the  struggle  for  existence 
for  a  time,  but  it  is  the  relatively  simple  organisa- 
tion which  endures. 


120  GEOLOGY 

The  Mollusca  are  represented  by  their  three  chief 
groups  in  the  earliest  fossiliferous  rocks.  The  two 
simpler  groups,  viz.,  the  "  Bivalves "  (including 
oysters,  cockles,  etc.)  and  the  "  Univalves  "  (or  snail 
group),  have  steadily  increased  in  variety,  complexity 
and  numbers  throughout  past  ages  (see  Plate  iv, 
figs.  5  and  6).  It  is  the  highest  group,  again, 
represented  now  by  the  cuttle-fish,  octopus,  and 
nautilus,  which  ran  through  its  development  most 
quickly,  and  is  to-day  in  a  very  decadent  condition. 
In  Palaeozoic  and  Mesozoic  times  these  "  Cephalo- 
pods"  were  much  more  abundant.  The  great 
family  of  Mesozoic  cephalopods — the  Ammonites — 
are  probably  the  most  familiar  and  popularly  prized 
of  all  fossils.  The  beauty  of  their  volute-like  shells 
is  seldom  excelled,  while  their  exceeding  abundance 
ensures  their  familiarity.  (See  Plate  iv,  fig.  3.) 

The  history  of  the  great  group  of  animals  to 
which  the  star-fish  and  sea-urchins  belong — the 
Echinodermata — is  very  fully  recorded  in  the  rocks. 
While  the  primitive  stock  consisted  of  more  or  less 
globular  or  egg-shaped  creatures,  covered  with  an 
irregular  mosaic  of  calcareous  plates,  they  developed 
along  a  variety  of  diverging  lines,  after  having  first 
become  a  typically  sedentary  group  and  acquired  in 
consequence  a  general  radial  symmetry.  Some, 
becoming  first  more  definitely  globular,  and  de- 
veloping great  regularity  in  the  arrangement  of 
their  plates,  arming  themselves  all  over  at  the  same 
time  with  an  elaborate  defence  of  spines,  gave  rise 
to  the  existing  group  of  the  sea-urchins,  which  in 
later  times  have  in  many  cases  lost  their  spherical 
form,  becoming  more  or  less  flattened  or  even 
biscuit-shaped  (see  Plate  iv,  fig.  4).  Another  series, 
having  many  of  their  covering-plates  reduced  to 


THE  HISTORY  OF  LIFE  ON  THE  EARTH      121 

mere  nodules  in  the  skin,  and  developing  also  five 
finger-like  lobes  or  "  arms  "  to  the  body,  became  the 
starfishes.  But  a  much  more  striking  series  of 
modifications  began  with  the  attachment  of  some 
forms  to  the  rocks  of  the  sea-bed  by  means  of  a 
flexible  jointed  stalk.  Subsequently,  these  attached 
forms  developed  a  set  of  five  jointed,  slender,  and 
often  much-branched  feathery  arms,  the  whole 
creature  thus  becoming  very  plant-like  in  appearance. 
A  few  of  these  "  sea-lilies "  or  Crinoids  still  exist, 
mostly  in  the  deep  sea — the  refuge  of  many  "  living 
fossils  " — but  they  reached  the  height  of  their  glory 
in  Palaeozoic  times,  and  not  a  little  of  the  Carboni- 
ferous limestone  of  the  Pennine  Chain  is  mainly 
composed  of  their  jointed  stems. 

Passing  now  over  the  remainder  of  the  lower 
forms  of  life,  let  us  turn  to  the  higher  creatures, 
whose  geological  history  must  ever  be  of  the  greater 
interest  to  the  palaeontologist  and  to  the  student  of 
evolution.  This  must  be  the  case  for  two  reasons : 
in  the  first  place,  the  history  of  the  higher  animals 
is  or  will  be  known  from  the  beginning:  while  the 
great  groups  of  the  lower  animals  had  all  been 
evolved  before  the  period  at  which  the  geological 
record  begins  to  be  legible,  so  that  we  can  only 
trace  the  details  of  their  subsequent  modification. 
The  starting  point  of  each  of  the  higher  groups, 
from  the  fishes  onwards,  lies  within  the  range  of 
known  geological  history.  Secondly,  the  structure 
of  these  more  advanced  forms  can  be  known  with 
much  greater  completeness.  In  the  case  of  the 
lower  animals,  the  only  part  capable  of  preservation 
in  the  fossil  state  is  usually  some  form  of  shell,  or 
external  covering,  which  as  a  rule  conveys  little 
information  as  to  the  structure  of  the  real  living 


122  GEOLOGY 

tissues  of  the  animal.  On  the  other  hand,  as  every- 
one knows,  the  higher  forms  possess  an  internal 
skeleton,  consisting  of  a  skull,  back  -  bone,  and 
supports  for  the  limbs ;  and  the  bones  of  this  skeleton 
are  related  so  intimately  to  the  structures  around 
them  that  it  is  possible,  from  their  study,  to  recon- 
struct almost  every  detail  of  the  animal's  body. 
The  muscles,  nerves,  and  blood-vessels  can  all  be 
restored  with  more  or  less  completeness,  and  the 
dry  bones  made  to  live  again. 

The  fishes  present  many  points  of  interest  in  their 
past  history.  The  gradual  elaboration  of  their 
scales,  teeth,  fins,  tails,  and  general  skeleton  can  all 
be  more  or  less  perfectly  traced,  though  most  of 
these  features  require  some  technical  knowledge 
for  their  comprehension.  Being  much  the  most 
primitive  of  the  vertebrate  animals,  the  fishes  are 
naturally  the  first  to  appear  in  order  of  time — viz., 
during  the  Silurian  period.  In  the  same  connection 
it  is  of  interest  to  note  that  the  simplest  group  of 
the  fishes  is  the  earliest  to  be  developed  and 
elaborated.  The  class  which  contains  the  sharks, 
dog-fish,  and  skates  is  in  many  respects  more 
primitive  than  the  great  mass  of  living  fish.  While 
the  majority  of  fish,  like  all  higher  animals,  have  a 
skeleton  of  bone,  the  shark  class  has  only  cartilage, 
which  never  becomes  replaced  by  bone  as  in  the 
higher  forms;  and  it  is  to  this  cartilaginous  or 
"shark"  class  that  nearly  all  the  early  fossil  fish 
belong.  In  the  Palaeozoic  period  they  formed  the 
dominant  group,  while  it  was  only  in  the  Mesozoic 
period  that  the  "  bony  "  fish  became  abundant.  Of 
still  greater  interest  is  an  even  more  primitive  group 
of  so-called  fishes — for  it  may  be  doubted  whether 
they  ought  to  be  classed  as  fish  at  all — the  hags  and 


THE  HISTORY  OF  LIFE  ON  THE  EARTH      123 

lampreys.  This  little  group  is  the  sole  remnant  of 
a  class  of  primitive  vertebrates  which  had  a  short 
but  brilliant  career  during  the  Silurian  and  Old  Red 
Sandstone  periods,  while  fish  life,  if  we  may  so  put 
it,  was  still  in  a  very  experimental  stage.  Seldom 
has  any  group  of  creatures  produced  more  truly 
archaic-looking  forms  than  these  relatives  of  the 
hag-fish  in  the  Old  Red  Sandstone  waters.  Yet 
many  of  the  other  contemporary  fish  were  little  less 
remarkable,  the  entire  Old  Red  Sandstone  group 
forming  a  most  fascinating  collection.  Those  fish 
enticed  Hugh  Miller  to  geology,  and  the  reader  may 
still  turn  to  his  "  Old  Red  Sandstone  "  and  "  Foot- 
prints of  the  Creator  "  for  the  most  lively  accounts 
of  them. 

Geological  history  is  still  silent  regarding  perhaps 
the  most  notable  event  in  the  history  of  life  on  the 
earth.  We  can  only  surmise  that  sometime  previous 
to  the  Carboniferous  period,  probably  during  the 
preceding  Devonian  times,  some  type  of  fish  ac- 
quired the  power  of  breathing  the  oxygen  in  the  air, 
as  well  as  that  dissolved  in  the  waters,  and  at  the 
same  time  had  its  paired  fins  modified  into  true 
limbs  which  enabled  it  to  crawl  on  to  the  land. 
Meantime  we  must  patiently  wait  the  day  when  some 
fortunate  blow  of  the  hammer  shall  disclose  the 
remains  of  these  forerunners  of  the  amphibia, 
reptiles,  birds,  and  mammals.  The  earliest  Car- 
boniferous and  perhaps  the  highest  Old  Red 
Sandstone  strata  contain  the  footprints  of  four- 
footed  creatures — the  first  traces  of  them  yet  found. 
Somewhat  higher  in  the  Carboniferous  rocks  the 
first  skeletons  of  the  amphibia  occur — salamander- 
like  creatures,  of  very  varied  size.  In  many 
respects  these  early  amphibia  were  more  primitive 


124  GEOLOGY 

than  their  living  descendants.  In  particular,  many 
of  them  appear  to  have  breathed  during  youth  by 
means  of  external  gills,  like  the  tadpole  of  the  frog. 
They  show  also  many  other  curious  points  of 
resemblance  to  certain  of  the  Old  Red  Sandstone  fish 
in  the — form  of  their  teeth,  the  development  of 
certain  protective  plates  in  the  eye,  the  arrangement 
of  the  bones  in  the  skull,  and  the  presence  of  scaly 
plates  over  a  portion  of  the  body — resemblances  which 
can  scarcely  be  altogether  accidental.  While  some  of 
these  early  amphibia  were  no  bigger  than  a  modern 
newt,  others  became  very  large  and  massive,  one 
having  a  skull  no  less  than  four  feet  in  length. 

The  most  highly  developed  of  the  Palaeozoic 
amphibia  grade  almost  imperceptibly  into  the  most 
primitive  of  the  reptiles,  which  make  their  first 
appearance  in  the  Permian  rocks.  This  transition 
from  the  amphibia  to  the  reptilia  is  made  doubly 
interesting  by  the  fact  that  the  reptiles  themselves 
almost  immediately  gave  rise  to  forms  which  present 
the  most  remarkable  resemblances  to  the  mammalia 
in  almost  every  part  of  their  anatomy,  resemblances 
so  strikingly  complete  that  some  of  the  best  authori- 
ties now  believe  that  these  primitive  reptiles  were 
the  direct  ancestors  of  the  mammals.  To  this  point, 
however,  we  must  return  later. 

The  history  of  the  reptiles  is  undoubtedly  the 
most  romantic  of  any  group  of  organisms.  The 
person  whose  knowledge  of  the  group  was  confined 
to  the  living  crocodiles  and  alligators,  tortoises, 
turtles,  snakes,  and  lizards,  would  little  guess  that 
the  reptiles  had  once  attained  a  dominance  over  the 
world  such  as  no  other  group  of  animals  has  ever 
approached.  Immediately  on  their  appearance  in 
the  world  they  evinced  a  most  wonderful  capacity 
for  modification  and  adaptation,  and  so  gave  rise  to 
forms  suited  to  every  kind  of  environment  and  mode 
of  life.  Fleet,  agile  forms  more  delicate  than  the 
cat  or  the  deer  were  associated  with  powerful 
massive  creatures  more  reminiscent  of  the  hippo- 


THE  HISTORY  OF  LIFE  ON  THE  EARTH      125 


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126  GEOLOGY 

potamus  or  the  elephant — and  the  variation  in  size 
was  much  greater.  Several  groups  became  modified 
for  amphibious,  and  ultimately  for  completely 
aquatic  life.  On  the  other  hand,  one  group,  the 
Pterodactyls,  developed  a  bat-like  form  and  gave 
rise  to  a  large  number  of  flying  reptiles.  Two 
groups  of  these  creatures  quite  overshadow  all  the 
others  in  the  magnificence  of  their  development. 
Some  of  the  Deinosauria  have  long  been  familiar  to 
English  readers  from  the  classic  descriptions  by  Dr. 
Mantell  of  the  remains  of  Iguanodon,  Hylaeosaurus, 
and  other  forms  from  the  rocks  of  Tilgate  forest  in 
Sussex.  But  perhaps  few  persons  would  be  more 
surprised  than  Mantell,  could  he  return  to  review 
the  Deinosaurs  now  known.  The  name,  which 
signifies  "terrible  reptiles,"  was  appropriately  ap- 
plied to  the  forms  first  known — reptiles  ten  or 
fifteen  feet  in  height,  and  much  more  in  length. 
How  much  more  appropriate  would  it  have  seemed 
had  the  American  Brontosaurus,  sixty  feet  in  length, 
been  then  known ;  or  the  still  larger  Atlantosaurus  ? 
We  need  scarcely  add  that  they  were  the  largest 
land  animals  which  have  ever  existed.  And  yet  the 
same  group  includes  the  fleet  little  creatures  no 
larger  than  a  rabbit,  already  referred  to.  It  would 
be  quite  impossible  to  do  justice  to  this  wonderful 
group  without  lengthy  descriptions  and  numerous 
illustrations.  They  were  the  land  animals  par 
excellence  of  the  Mesozoic  age.  The  great  size  of 
many  of  them,  the  wonderful  variety  of  horns  and 
scaly  armour  with  which  many  were  provided,  have 
well  merited  them  the  appellation  "  the  dragons  of 
their  prime."  While  the  largest  forms  were  adapted 
to  feed  on  the  vegetation  of  the  period,  they  were 
kept  in  check  by  the  scarcely  less  formidable 
carnivorous  species. 

Little,  if  at  all,  inferior  to  the  Deinosaurs  in 
abundance,  variety,  and  size  were  the  great  marine 
reptiles  belonging  to  the  groups  of  the  long-necked 
Plesiosauria  and  whale-like  Ichthyosauria,  which  are 


THE  HISTORY  OP  LIFE  ON  THE  EARTH      127 

probably  the  most  familiar  of  all  fossil  vertebrates. 
As  the  Deinosaurs  dominated  the  land,  so  they  must 
have  been  supreme  in  the  oceans.  There  is  a 
striking  analogy  between  these  great  marine  reptilia 
and  the  whales  among  the  mammals.  Both  attained 
enormous  size,  the  Plesiosaurs  rivalling  the  whales 
in  this  respect,  both  had  their  structure  modified  in 
a  similar  manner  for  aquatic  life,  both  were  de- 
scended from  comparatively  small  land-dwelling 
ancestors. 

The  ancestors  of  the  true  crocodiles  and  alligators 
also  existed  in  the  Mesozoic  seas — or,  rather,  on  the 
shores,  and  were,  on  the  whole,  remarkably  similar 
to  the  modern  forms  in  their  structure.  In  general 
appearance  and  in  habits,  they  were  probably 
scarcely  to  be  distinguished  from  their  living 
representatives.  True  tortoises  also  appeared  as 
early  as  the  Triassic  period,  with  their  typical 
shield-like  armour.  The  snakes  and  lizards  are 
scarcely  known  among  fossil  forms;  but  in  the 
Cretaceous  period  there  existed  a  group  of  marine 
reptiles  closely  allied  to  them — true  "  sea-serpents  " 
in  fact,  and  some  of  these,  again,  attained  enormous 
size,  probably  sixty  feet  in  length. 

Far  from  the  least  remarkable  feature  in  the 
history  of  the  reptiles  is  their  sudden  extinction  at 
the  end  of  the  Mesozoic  era.  In  the  Cretaceous 
period  the  Plesiosaurs  and  Ichthyosaurs  were,  it  is 
true,  already  in  a  decadent  condition,  but  they  were 
replaced  by  the  no  less  formidable  sea-snakes.  The 
Deinosaurs  were  represented  by  their  largest  forms, 
and  the  flying  reptiles  likewise  included  species 
measuring  no  less  than  twenty-seven  feet  across  the 
wing.  The  reptiles,  in  fact,  were  undisputed  masters 
of  air,  sea,  and  land.  Yet,  without  any  obvious 
cause,  all  these  dominant  groups  died  out  simultan- 
eously, leaving,  at  the  dawn  of  the  Tertiary  era, 
only  the  same  contemptible  group  of  "creeping 
things  "  which  are  the  reptiles  of  to-day. 


128  GEOLOGY 

The  history  of  the  birds  is  very  lightly  sketched 
by  their  fossil  remains.  What  there  is  of  it,  how- 
ever, is  of  great  interest.  The  earliest  known  bird, 
Archaeopteryx,  appears  only  in  the  middle  of  the 
Mesozoic  era — in  the  Corallian  rocks  of  Germany. 
Outwardly  a  typical  bird  in  the  possession  of  a 
perfectly  normal  coat  of  feathers,  closer  inspection 
readily  reveals  a  number  of  deeply  significant 
features.  In  place  of  a  horny  beak,  the  jaws  are 
armed  with  simple  rows  of  conical  teeth,  set  in 
sockets  in  the  normal  manner.  The  wings  possess 
three  (possibly  four)  well  developed  grasping  fingers 
armed  with  claws,  as  well  as  the  one  greatly  en- 
larged digit  which  carries  the  wing  feathers.  But 
perhaps  the  most  remarkable  feature  of  all  is 
the  long  graceful  tail,  which,  instead  of  consisting 
of  a  simple  bunch  of  feathers,  as  in  all  living  birds, 
was  rather  the  tail  of  a  reptile,  an  extensive  flexible 
prolongation  of  the  back-bone,  each  joint  of  which 
carried  a  pair  of  feathers;  surely  as  incongruous  a 
combination  of  characters  as  nature  ever  produced. 
Indeed,  in  all  characters  in  which  Archaeopteryx 
differs  from  the  living  birds,  it  assimilates  to  the 
reptiles,  and  points  unmistakably  to  the  reptilian 
ancestry  of  the  former.  Curiously  enough,  how- 
ever, it  was  not  to  the  flying  reptiles  that  the  birds 
were  immediately  related,  but  rather  to  the  Deino- 
saurs,  many  of  which  presented  several  bird-like 
modifications.  Up  to  the  end  of  the  Mesozoic  era 
the  birds  retained  the  teeth  in  their  beaks,  but  the 
other  special  marks  of  their  reptilian  descent 
speedily  disappeared. 

The  history  of  the  mammals  is  peculiar.  We 
have  already  had  occasion  to  refer  to  the  quite 
extraordinarily  mammal-like  character  of  some  of 
the  early  reptiles  of  the  Permian  and  Triassic  rocks 
which  lead  to  a  strong  suspicion  that  the  mammals 
are  descended  from  them  or  their  near  relations. 
Now  the  earliest  mammalian  remains  are,  in  fact, 
found  in  the  highest  part  of  the  Triassic  rocks ;  yet 


THE  HISTORY  OF  LIFE  ON  THE  EARTH      129 

they  are,  as  far  as  the  meagre  evidence  indicates, 
far  from  being  the  kind  of  mammal  we  should  have 
expected  to  arise  from  the  reptiles  referred  to. 
They  are  puny  creatures,  of  very  primitive  type, 
about  the  size  of  a  rat.  And  these  little  beasts  are 
the  only  known  forms  throughout  the  Mesozoic  era. 
During  the  early  part  of  the  Cainozoic  era,  in 
Europe  and  in  America,  the  principal  types  of  the 
mammalia  appear  in  rapid  succession,  the  majority 
of  forms  showing  no  special  relationship  to  the 
little  creatures  already  referred  to,  or  any  other 
indication  of  their  ancestry.  These  facts  have  been 
explained  on  the  assumption  that  we  are  dealing 
with  a  group  of  immigrants  from  some  unknown 
region,  whose  advent  was  now  made  possible  by  the 
extensive  geographical  changes  which  marked  the 
close  of  the  Mesozoic  and  dawn  of  the  Cainozoic 
eras.  If  true,  until  that  unknown  region  is  dis- 
covered, the  history  of  the  early  evolution  of  the 
mammalia  must  remain  unread. 

When  the  mammals,  then,  for  all  practical  pur- 
poses, first  appeared  in  Europe  and  America,  the 
great  families  were  already  marked  out.  Neverthe- 
less, as  by  way  of  compensation,  the  story  of  their 
subsequent  development  is  preserved  with  wonder- 
ful completeness,  and  we  already  know  in  great 
detail  the  history  of  the  development  of  several 
important  types. 

The  story  of  the  horse  has  become  classic.  That 
familiar  animal  is  one  of  the  most  specialised  of  the 
living  mammalia,  at  least,  as  regards  the  structure 
of  its  limbs.  The  so-called  "  knee  "  of  the  horse  is 
really  its  ankle  or  wrist,  the  "shin"  or  "cannon"  bone 
being  the  first  or  "  palm  "  joint  of  the  single  finger 
or  toe  which  each  limb  possesses,  while  the  hoof  is 
of  course  the  enlarged  "nail"  covering  the  last  joint. 
A  greater  contrast  could  scarcely  be  imagined  than 
that  between  this  lower  portion  of  the  limb  of  a  horse 
and  the  hand  01  foot  of  a  man — which  latter  repre- 
sent the  normal  mammalian  type.  Yet,  as  we  trace 

i 


130 


GEOLOGY 


FIG.  12A.— DEVELOPMENT 
OF  THE  SKULL  OF  THE 
ELEPHANT. 

After  Andrews. 

1.  Moeritherium 

— Middle  Eocene. 

2.  Palceomastodon 

— Upper  Eocene. 

3.  Tetrabelodon 

— Miocene. 

4.  Tetrabelodon 

— Miocene  and  Pliocene. 

5.  Elephas 

— Pliocene  and  Living. 


FIG.  12B. — DEVELOPMENT  OF 
THE  LIMBS  AND  TEETH  OF 
THE  HORSE. 

1.  Orohippus — Eocene. 

2.  Mesohippus — Miocene. 

3.  Miohippus — Miocene. 

4.  Protohippus — Pliocene. 

5.  Pliohippus — Pliocene. 

6.  Equus — Living. 

N.B.— There  should  really  be  a 
gradual  increase  in  size  from  1  to  6 
as  noted  in  text. 


THE  HISTORY  OF  LIFE  ON  THE  EARTH      131 

the  ancestors  of  the  horse  back  stage  by  stage 
through  the  successive  stages  of  the  Cainozoic  rocks, 
we  find  the  specialisation  of  its  limbs  becoming 
gradually  less  marked  till  an  almost  perfectly  normal 
type  is  reached,  in  a  manner  which  is  conveyed 
much  more  clearly  by  the  accompanying  illustration 
than  it  could  be  by  any  description.  (See  fig.  12b.) 
Along  with  these  modifications  in  the  limbs  other 
changes  proceeded ;  the  size  of  the  animal,  which, 
to  begin  with  was  no  larger  than  a  collie,  gradually 
increased,  and,  more  notably,  the  teeth  gradually  in- 
creased in  complexity  from  a  very  simple  form  to 
their  present  highly  specialised  condition. 

More  recently  the  history  of  another  very  special- 
ised group  of  mammals  has  become  very  completely 
known — that  of  the  elephants.  Until  a  few  years 
ago  we  only  knew  the  latter  part  of  their  story, 
because  the  earlier  history  was  buried  in  the  rocks 
of  Egypt.  The  elephant  shows  its  specialised 
character  partly  in  its  great  size,  but  more  particu- 
larly in  its  highly  modified  teeth  and  jaws.  Besides 
the  pair  of  enormously  developed  teeth  which  form 
the  tusks  of  the  ordinary  elephant,  there  is  at  any 
one  time  on  one  side  of  each  jaw  only  one  other 
large  tooth  to  be  seen,  or  at  most  a  portion  also  of  a 
second  one.  Nevertheless  the  size  of  each  tooth  is 
such  that  it  almost  completely  fills  the  side  of  the 
jaw;  and  the  complexity  of  the  teeth  is  commen- 
surate with  their  size.  The  "crown  "  is  enormously 
high,  and,  instead  of  bearing  merely  a  few  tubercles 
on  the  surface,  as  a  normal  tooth,  these  are  modified 
into  large  transverse  vertical  plates  ranged  com- 
pactly one  behind  the  other  to  the  number  of  over 
twenty,  the  whole  forming  the  most  efficient  tooth  for 
grinding  vegetable  material  that  Nature  has  ever 
devised.  Again,  though  but  one  tooth  at  a  time  ap- 
pears on  each  side  of  the  jaw,  the  animal  during  its 
life  has  several  such  teeth  successively  in  the  same 
position.  These  successive  teeth,  however,  do  not 
represent  successive  "sets"  in  the  proper  sense,  but 


132  GEOLOGY 

are  all  members  of  the  one  permanent  set,  which, 
unable  from  their  enormous  size  to  appear 
simultaneously,  have  been  forced  to  become  succes- 
sional.  Traced  back  to  the  Eocene  rocks  of  Upper 
Egypt,  the  elephants  are  found  to  have  their 
descent  from  a  small  animal  which  has  been  named 
Moeritherium,  a  creature  about  the  size  of  a  pony, 
with  an  almost  perfectly  typical  set  of  low-crowned 
tuberculate  teeth,  and  no  tusks  more  striking  than 
those  of  a  modern  boar.  In  the  later  Eocenes, 
larger  forms  occur  with  more  prominent  tusks  (in 
both  lower  and  upper  jaws),  with  the  "front"  or 
incisor  teeth  already  obliterated,  and  the  "  chin  "  of 
the  lower  jaw  elongated  in  the  direction  of 
the  forwardly-projecting  tusks,  so  as  to  form, 
with  them,  a  kind  of  "  spade."  Evolution  con- 
tinued in  the  same  direction  to  the  early  Miocene 
period,  when  the  chin  was  extraordinarily  elongated, 
and  the  teeth  larger  and  correspondingly  less 
numerous.  Subsequently  the  chin  became  again 
reduced,  and  the  nose  began  to  develop  into  the 
familiar  "  trunk,"  while  the  line  of  descent  leading 
to  the  modern  elephants  branched  off  in  forms 
possessing  tusks  only  in  the  upper  jaw.  All  the 
while  the  teeth  grew  larger  and  more  complex,  the 
tusks  in  particular  became  enormously  developed, 
as  also  the  trunk,  till  the  line  reached  its  maximum 
development  in  forms  including  the  recently  extinct 
Mammoth,  some  of  which  were  much  greater  in 
bulk  than  any  living  elephant. 

Finally,  one  other  line  of  mammalian  descent 
may  be  briefly  referred  to — that  of  the  deer.  This 
line  presents  many  features  in  common  with  that  of 
the  horse,  to  which  it  is  somewhat  closely  allied. 
The  teeth  present  a  practically  identical  evolution, 
albeit  it  has  never  reached  so  advanced  a  stage. 
The  limbs  also  have  specialised  in  a  similar  manner, 
with,  however,  this  important  difference :  that 
instead  of  only  one  digit  having  persisted,  two  are 
preserved;  but  this  pair  of  digits  has  almost 


THE  HISTORY  OF  LIFE  ON  THE  EARTH       133 

completely  fused  together,  thus  giving  the  same 
effect  as  in  the  horse,  and  exemplifying  the 
oft-repeated  truth  that  Nature  may  attain  the 
same  end  by  quite  diverse  means.  It  is  to  the 
history  of  the  antlers,  however,  that  we  wish 
to  refer.  The  earliest  known  deer  occur  in  the 
upper  Eocene,  but  are  quite  devoid  of  antlers ;  and 
not  till  the  later  Miocene  period  were  these 
developed.  When  they  did  appear  they  were  quite 
simple  in  form,  with  a  single  prong  or  "  tine."  As 
we  trace  the  deer  through  their  later  development 
we  find  the  number  of  tines  gradually  increasing, 
and  the  whole  antler  becoming  larger  and  more 
complex,  till  it  reached  its  maximum  in  the  magnifi- 
cent crown  of  the  Deer  found  fossil  in  the  Irish 
peat-bogs,  with  antlers  ten  feet  across.  Now,  as 
everyone  knows,  this  history  of  antler  development 
is  repeated  by  every  deer  in  the  course  of  its  own 
life.  Its  first  pair  of  antlers  is  simple,  but  each 
successive  pair  becomes  more  complex  till  it  attains 
complete  maturity  and  becomes  a  royal  stag.  Here 
we  have  a  striking  illustration  of  the  fact  that  an 
animal,  in  the  course  of  its  development,  repeats  to 
some  degree  the  stages  of  evolution  through  which 
its  ancestors  have  passed — it  "climbs  its  family  tree" 
— and  the  study  of  development,  Embryology, 
materially  aids  us,  therefore,  to  supplement  and 
interpret  the  fragmental  palaeontological  history 
of  life. 

Here  we  must  conclude  these  brief  "  chapters  " 
out  of  the  vast  history  of  the  earth,  with  the  hope 
that  they  may  stimulate  the  reader  who  has  come 
thus  far  to  go  a  little  further. 

THE    END. 


INDEX 


PAGE 
.  57 
120 
123 
124 
133 
114 
.  128 
.  114 
.  125 


91 
12S 
120 

49 


Acids,  organic,  in  soil    . . 

Ammonites 

Amphibia,  first  occurrence  of 

relation  to  fishes 

Antters,  history  of,  in  deer 
Antrim,  volcanic  rocks  . . 

Archaeopteryx       

Arran,  volcanic  rocks 

Atlantosaurus        

Balance  of  continental  masses 

Basalt 

Ben  Lawers,  section  of  . . 

Birds,  fossil 

Bivalves  (Pelecypoda)    .. 
Bombs,  volcanic 

Bothnia,  Q-ulf  of,  movement  of 

land         •  •    8» 

Brachiopoda,  fossil  ..  -.119 
Bridlington  Bay,  encroachment 

of  sea .-68 

British  Isles,  geological  map  of  104 

submerged 

plateau  of 

Brontosaurus  .  •  •  •  125, 126 
Caderldris,  volcanic  rocks  of..  108 
Cainozoic  

mammalia  of . . 

Cambrian  rocks 

Carbonic  acid,  action  on  rocks 

in  sea  water     . . 

Carboniferous  rocks 

Cemeteries,  weathering  ex- 
hibited in         

Cephalopoda          

Chalk  period,  geographical 
conditions         

Chalk,  origin  of 

Circumference  of  Earth,  early 
measures          

Climatic  changes  in  Cainozoic 

Coal  Measures       

as  proof  of 
subsidence    . . 

Comets          

Cone  of  volcano,  origin  of        . . 

"Continental"  periods  of 

Britain    . .         . .        105, 110, 113 

Cooling  of  lava  and  size  of 
crystals 

Copernicus,  discovery  of  true 
nature  of  Solar  System 

Corals,  fossil 

"precious" 

Coral-reefs 

Cretaceous  reptiles 

Crevasses,  engulfing  moraines 

Crinoids  (Sea-lilies) 

Crocodiles,  fossil 

Crystalline  rocks 

Crystallisation  of  lavas  . . 

Cumbrae— Lion  Rock     . . 


100 
129 
106 
64 
77 
110 

63 

120 

112 

78 

5 

114 
110 

83 
21 
32 


49 

6 

118 
119 

73 
127 

61 
121 
127 

45 


51 


PAQB 
..  52 
..  71 
.  56 


Cullin  Hills,  Skye  .. 
Currents  in  oceans 
Danube,  sediment  carried  by 
Darwin,  Sir  G.,  on  tides  and 

rigidity  of  earth       . .        . .    36 
Deep  sea        75 

life  of       76 

Deer,  history  of 132 

Deinosaurs 126 

Deltas,  formed  in  lakes  . .        . .    55 
Denudation 69 

estimated  from 

folds 92 

Depth  of  Sea,  distribution  of 

sediments         71 

Deserts,  origin  of 62 

Devonian  rocks 109 

Dip  of  rocks 88 

Disintegration  of  rocks  . .  . .  57 
Dolomite,  origin  of  . .  . .  76 

Dykes 51 

Earth,  weight  of 24 

compared  with  surface 

rocks          26 

Earth-movements,  cause  of    . .    95 

in  N.  W. 

highlands..  103 
Earthquakes  ..        ..          36-40 

accompanying 

eruptions  ..        ..28 

displacements  of 

ground  in  . .         37,  86 

effects  of    . .        . .    36 

extent  9f  move- 
ments in    . .        . .    39 

.        recording  instru- 
ments        ..         ..38 

Echinodermata 120 

Elephants,  history  of  . .  . .  131 
Elevation  of  land,  proofs  of  . .  81 
Eocene  period,  geographical 

changes 113 

Estuary,  deposits  formed  in    . .    65 

of  Lower  Oolites       . .  112 

of  Lower  Cretaceous    112 

Faults 92 

rate  of  formation  of      . .    93 

Fishes,  history  of  the     . .        . .  122 

Folding  of  rocks 89 

Footprints,  earliest  known 

fossil        123 

Footprints  in  Triassic  rocks  . .  Ill 
Foraminifera,  fossil  . .  .  118 

in  deep-sea  ooze  76,  77 

Fossils  as  guides  in  arrange- 
ment of  rocks 99 

Fossils,  distribution  of  . .  . .  116 
Frost,  action  on  rocks  and  soil  58 

Gabbros         52 

Gastropoda  (Univalves)  ..120 


INDEX 


135 


PAGE 

Geological  Map  of  British  Isles  104 

Glacial  period         114 

Glaciers          60 

erosion  by          . .        . .    61 

Glen  Tilt,  Button's  observa- 
tions in 47 

Grampian  hills,  rocks  of          . .  105 

structure  of    90,  91 

Granite          44 

microscopic  section  of     45 

weathering  of     . .        . .    64 

"Hag-fish,"  fossil..         ..      122,123 
Heat  as  cause  of  volcanic  action    34 

of  earth's  interior  . .        . .    27 

solar,  in  destruction  of 

rocks  57 

Hipparchus,  estimate  of  sun's 

distance 6 

Horse,  history  of  . .        . .     130, 131 
Hutton,  the  igneous  origin  of 

granites 

Hyleeosaurus 

Ice,  action  on  rocks  and  soil 

Ice-age          

Ichthyosauria 

Iguanodon    

Internal  heat  of  earth    . . 
Interior  of  earth,  probable 

condition 
Joints,  importance  in  marine 

erosion 

Jupiter 
belts  of       .. 

temperature  of 

varying  rotation 

Krakatoa,  eruption  of    . . 
Lakes,  deltas  formed  in . . 
Lake  District,  volcanoes  of 
Land  surfaces  sunk  below  sea 
Land-vertebrates,  unknown 

origin 

Lava,  How-structure  in  . . 
microscopic  structure  of  45,48 


46 
125 

58 
114 
126 
125 

35 

41 


11-14 
..     12 

12,13 

.       13 

..     30 

..     55 

108 

84 

123 


Vesuvian,  nature  of        . .    52 

Lavas,  varying  composition  of  52 
Leasowe,  submerged  forest  . .  83 
Level  of  land,  changes  of  . .  81 

Lewisian  rocks       103 

Liassic  Sea 112 

Life  in  early  rocks 117 

Lime  in  ocean,  source  of  . .  73 
Limestone,  origin  of  . .  . .  73 
Lion  Rock,  Cumbrae  . .  . .  51 
London  basin,  section  of  . .  91 

Magma          53 

Magnesian  limestone,  origin  of  75 
Mammalia,  origin  of  . .  124, 128 

Mammoth 132 

Mars 9-11 

atmosphere  of     *  . .        , .    11 

canals  of        10 

snow-cap  of 10 

Martinique,  history  of  erup- 
tions in 27 


PAGE 

Meanders,  action  of  rivers  in  66, 67 

Mesozoic       100 

—     reptiles 186 

rocks       ..        ..     111,118 

Metamorphism      ..        ..        ..94 

Meteorites 21 

Miller,  Hugh,  on  fossil  fish     . .  123 

Millstone  Grit        110 

Miocene,  earth-movements  in    113 

volcanic  activity  in    . .  114 

Mississippi  sediment  carried  by    56 

Moaritherium         132 

Molten  stage  of  all  rocks         . .    53 

of  earth's  crust . .  54 

Moraines       61 

Mountains,  disintegration  of 

rocks 59 

soil  absent  from  ..  59 

variation  of  tem- 
perature   ..        ..59 
Mountain  ranges,  structure  of  90 
Movement  of  land,  rate  of       . .  86 

Mud,  origin  of       74 

Mull,  volcanic  rocks       ..        ..114 

Nebul»         20 

evolution  of       . .        . .  22 

relation  to  comets  and 

meteorites       ..       ..22 

Neck,  volcanic       61 

Neptune,  discovery  of  ..        ..     8 

"Neptunista"        46 

North  Sea,  deposits  in  ..  ..70 
North  Wales-raised  beaches  of  82 
North- West  Highlands,  rocks  of  103 
Nuovo,  Monte,  history  of  . .  32 
Old  Red  Sandstone  . .  . .  109 
Old  Red  Sandstone,  fish  of  . .  123 

Oo«e  of  deep  sea 75 

Ordovician  period,  land  and 

sea  of 107 

rocks 108 

volcanoes      ..        ..  108 

Organic  acids  in  soil       . .        . .    57 
Outer  Hebrides,  rocks  of         . .  103 
Palaeontology,  bearing  on 

evolution          lie 

Palaeozoic      . .        . .         . .        . .  100 

Paris,  t'oraminiferal  limestones  118 
Pelecypoda  (Bivalves)    . .        . .  120 

Pelee,  Mount,  eruptions  of     . .    28 

Pennine  Chain,  origin  of         . .  110 

structure  of   . .    90 

Permian  rocks        in 

Planets,  distance  from  sun      . .      8 

masses 8 

densities 8 

minor        8 

origin  of  ..        ..         22,23 

Pleistocene 114 

Plesiosauria 126 

Pliny,  account  of  eruption  of 

Vesuvius          30 

Pliocene        114 

Plutonic  rocks       50 

"Plutoniste"         47 


136 


INDEX 


PAGE 

Pre-Oambrian  period,  length  of  117 
Preliminary  tremors  of  earth- 

quakes    ......         39,40 

-  evidence  of  rigid 

interior  of  earth       .  .        .  .    40 

Pterodactyls  (flying  reptiles)  125,127 
i'tolemaic  system  ......      6 

Radiolaria  in  deep-sea  deposits    79 
Raised  beaches      ......    82 

Recapitulation  in  development  133 
Red  Clay       .......      79 

Reptiles,  dominance  of  .  .  .  124 

-  early  forms  of  .  .        .    124 

-  extinction  of    ..        .127 
Rhyolite        .......      52 

-  microscopic  section  of    45 
Rivers,  action  of    .....      66 

-  changes  of  course       .      67 

-  lowland     .  .        .  .         .      66 
Roches  moutonnees        .  .        .61 
Roots,  aid  in  formation  of  soil      57 
Salts  of  sea,  source  and  fate  of     75 
Salt  in  Permian  and  Triassic 

rocks        ........  Ill 

Sand,  origin  of       .....     74 

Schists,  etc.,  of  Grampians     .    106 
Screes  ..        ..        .....      59 

Sea-bed,  distribution  of  sedi- 

ments on  .......      71 

Sea,  erosion  of  coast  by  .  .        .      68 
Sea-lilies  (Orinoids)        ..        .121 
Sea-urchins  ........  120 

Sedimentary  rocks,  derived 

from  igneous    53 

-  relation  to  igneous    74 
Sediments,  distribution  of      .  .    71 
Sediments  prove  slow  subsi- 

dence      ........    86 

Seismographs         .  .        .  .         38,  39 

Seismology   ........    38 

Sharks,  among  earliest  fish  .  .  122 
Shooting-stars  .  .  .  .  .  21 

-  in  deep-sea  deposits    80 


Silurian  rooks 

Simple  organisms  survive       .  . 

Skye,  Cullin  Hills  ...... 

-  volcanic  rocks       .  .        .  . 

Slate,  origin  of       ...... 

Snow,  protecting  rocks  .  .        .  . 

Snowdon,  volcanic  rocks  of     .  . 

Spectroscope,  applied  to  study 

of  Sun      ........ 

Soil,  origin  of         ...... 

-  waste  of         ...... 

Solar  heat,  in  deserts      .  .        .  . 

-       in    destruction    of 
rocks   ...... 

Solar  System,  table  of  elements 

-         unity  of    .  .        .  .    18 

Solution  of  shells  in  sea.  .        .  .    77 

Somma,  Monte       ......    33 

South-east  England,  section  of    91 
St.  Pierre,  destruction  of        .  .    29 
Stacks,  origin  of    ......    69 

Stars,  distances  of 


108 

119 
62 

114 
94 
60 

108 

17 
66 
56 
62 

57 


19 


PAQB. 
Stars,  evolution  of  ..        ..28 

types  of        20 

Star-fishes m 

Steam  in  volcanic  eruptions  . .  3d 
Stratified  rocks 42 

table  of  British  100 

Stromboli 31 

Submerged  forests          . .        . .    8* 
Subsidence  of  land,  rate  of    85,  87 

Subsoil 57 

Succession  of  rocks,  historical 

order       97 

Sulphurous  gases  in  eruptions    23 

Sun      14-18 

composition  of        ..        ..17 

distance  of 7 

gaseous  condition  . .        . .    16 

red  flames  (prominences)    16 

rotation  of 16 

spots  on,  nature  of . .        . .    16 

telescopic  appearance     . .    16 

temperature  of       . .        . .    14 

Survival  of  simple  organisms. .  119 
Sweden,  movement  of  land  in . .  88- 
Temperature  of  Earth's  interior  27 

gradient      . .        . .    27 

variation  affecting 

rocks . .         . .        . .    57 

Terrigenous  deposits      . .        . .    71 

Thrust  planes         9$ 

Tidal  currents        72 

Tides  proving  rigidity  of  earth  36 
Torridonian  sandstones  . .  105 

Transport  by  rivers       . .         65-68 

Triassic  rocks         Ill 

Tundras,  origin  of  . .        . .    58 

Unconformity  . .  . .  101, 102 
Univalves  (Gastropoda)  ..  120 

Uranus,  discovery  oy  Herschell  8- 
Valleys  sunk  below  sea  . .  . .  84 
Vertebrata,  knowledge  of  fossil  121 
Vertebrates,  unknown  origin 

of  land-  123 

Vesuvian  lava,  nature  of         . .    52 

Volcanic  rocks       5O 

of  Old  Red  Sand- 
stone       . .        . .  109 
Volcano,  ideal  section  of         . .    31 

origin  of  cone  . .        . .    32 

Volcanoes 27-35 

British 50- 

dissected  . .        . .    51 

distribution  of..        34,  3& 

of  Miocene  period     . .  114 

of  Ordovician  period  10* 

relation  to  mountain- 

ranges  33 

Water  given  out  by  volcanoes  34 
Waterfalls,  action  of  . .  . .  66- 
Waves.  action  on  cliff  . .  . .  6* 
Weathering  of  rocks  . .  . .  63 

—  exhibited  in  cemeteries  63 
Werner,  the  origin  of  Granites  46 
Wales,  rocks  of  . .  . .  106-108 

volcanoes  of  . .  108- 


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